Regarding Table III.1–1 (GCRB 199): What do the -1, -2, and -3 Encumbrance entries indicate? And how do they interact with the guidelines for armor encumbrance (shown on the same page)?

The way I would look at it is this: all armour starts at 0 Soak, 0 Defence, 1 Encumbrance, 0 Cost. Then go through the table and apply Defence and Soak modifiers, including the 3 options at the very bottom.

Looking at example equipment in the settings chapter Encumberance looks to be increased by 1 point per point of Soak, and extra for Defence if you combine Defense and Soak on the same item.

Then adjust the Encumbrance to suit the theme of the setting, take a look at Plate vs Magic armour on p145 for an example of increased cost for reduced Encumbrance.

Finaly you should look over the whole thing and decide if all the armour in your setting is evenly spread, and nothing looks too good for too little. The most important prices will be the low value (0-400) items since they are available at character creation, anything else will result from adventure rewards.

Was there a specific setting and armour piece you want to make?

Naturally. I am working on a list of high fantasy armor as a pet project, mostly inspired by later french and german designs. I had nine entries ideated, described, and ready to have mechanics assigned to them... and then I ran into a wall pricing the first one.

I could have just used existing entries as a basis, but since this list is intended to replace the existing options, their values weren't really relevent except as examples of the pricing metrics; which I was unable to reverse engineer in a reasonable period of time.

I expected some deviation, as the book admits they're just guidelines, but with your answer I was able to come much closer to an accurate analysis of the existing armor prices. More importantly you've provided an acceptable "zero-point" model for me to build up from for my armor set. Even if the math isn't perfect, at least all of the item's prices will be internally consistent.

This is all work towards a much larger project that I may or may not ever get around to. However if ya'll would like I will post the set once it is ready for critique.

NEW ITEM QUALITIES
Fortify (Passive)
While wearing armor or using an item with the Fortify quality, your character reduces any Critical Injury result they suffer by 5 times its Fortify rating, to a minimum of "01". Multiple sources of fortify stack with one another. For example, a character wearing armor with Fortify 3, and using a shield with Fortify 2 has a total rating of Fortify 5 (which reduces the result of any Critical Injury they suffer by 25)
Price: 50 × (Fortify rating)²

Noisy (Passive)
While wearing armor or using an item with the Noisy quality, your character adds a number of Setback to Stealth checks they make equal to its Noisy rating.
Price: -50 × (Noisy rating)²

NEW ARMOR SUITS
Heavy Robe
A heavy robe is made from several layers of thick fabric. They're usually worn for comfort or aesthetic, rather than the minimal protection they provide.
Armor Profile: 0 Defense, +1 Soak, 0 Hard Points, 2 Encumbrance. Price 25, Rarity 0.

Sturdy Jacket
A sturdy jacket is made from leather with a thin fabric lining. They're usually worn for comfort or aesthetic, rather than the minimal protection they provide.
Armor Profile: 0 Defense, +1 Soak, 0 Hard Points, 1 Encumbrance. Price 75, Rarity 0.

Winter Cloak
A winter cloak is made from wool or furs. Although typically worn for warmth, these durable cloaks also be used to deflect projectiles.
While wearing a winter cloak, your character removes one Setback from any Survival or Resilience checks they make due to cold weather, but add one Setback to any Survival or Resilience checks they make due to warm weather. A winter cloak can be worn over other armor, and its benefits stack with the other suit of armor while doing so.
Armor Profile: 0 Defense, 0 Soak, 0 Hard Points, 1 Encumbrance; Deflection 1. Price 25, Rarity 0.

Aketon
An aketon is a padded doublet made from layers of quilted fabric and leather. They're designed to be worn beneath a particular kind of heavier armor (such as a brigandine or maille ensemble). An Aketon is rarely worn seperately, except by a courtier donning a fashionable aketon in order to feign martial prowess. However, it also provides minimal protection by itself. An aketon includes a matching set of sturdy leather boots and gloves.
Armor Profile: 0 Defense, +1 Soak, 0 Hard Points, 1 Encumbrance. Price 75, Rarity 1.

Gambeson
A gambeson is a padded greatcoat made from layers of quilted fabric and soft leather. Although similar to an aketon, a gambeson is actually designed to be worn as armor. A gambeson includes a matching set of sturdy leather boots and gloves.
Armor Profile: 1 Defense, +1 Soak, 1 Hard Point, 2 Encumbrance. Price 200, Rarity 2.

Brigandine
A brigandine is an armored coat or vest made from small metal plates riveted between layers of soft leather or quilted fabric. A brigandine includes a matching aketon.
Armor Profile: 1 Defense, +1 Soak, 1 Hard Point, 2 Encumbrance; Fortify 1. Price 250, Rarity 4.

Cuir Bouilli Suit
A cuir bouilli suit is armor made from thick, intricately shaped, boiled leather plates, which are either riveted or strapped together. This armor protects the wearer's whole body. A cuir bouilli suit includes a matching aketon.
Armor Profile: 1 Defense, +2 Soak, 1 Hard Point, 2 Encumbrance. Price 825, Rarity 5.

Lacquered Ensemble
A lacquered ensemble is a suit of armor made from lacquered wooden splints and wicker meshs affixed to a soft leather backing with quilted fabric padding. This armor protects the wearer's whole body. A lacquered ensemble includes a matching aketon.
Armor Profile: 1 Defense, +2 Soak, 2 Hard Points, 2 Encumbrance; Fortify 1, Noisy 1. Price 875, Rarity 5.

Maille Ensemble
A maille ensemble is a suit of armor made from several layers of interlocking metal links and quilted fabric. A maille ensemble consists of a hauberk, chausses, maille-gauntlets, and a maille-coif. A maille ensemble includes a matching aketon.
Armor Profile: 2 Defense, +2 Soak, 2 Hard Points, 3 Encumbrance; Fortify 2, Noisy 2. Price 2,000, Rarity 6.

Scaled-Maille Suit
A scaled-maille suit is armor made from small, overlapping metal plates riveted to a layer of maille with a quilted fabric backing. A scaled-maille suit includes maille-gauntlets, an aventail helm, and a matching aketon.
Armor Profile: 2 Defense, +3 Soak, 2 Hard Points, 3 Encumbrance; Fortify 3, Noisy 2. Price 2,825, Rarity 7.

Plated-Maille Suit
A plated-maille suit is armor made from large overlapping metal plates riveted to a layer of maille with a quilted fabric backing. A plated-maille suit includes plated-gauntlets and sabatons, a great helm, and a matching aketon.
Armor Profile: 3 Defense, +3 Soak, 2 Hard Points, 4 Encumbrance; Fortify 3, Noisy 2. Price 4,250, Rarity 7.

Fully-Plated Ensemble
A fully-plated ensemble is a suit of armor made from dozens of intricately shaped, thickly padded, articulated metal plates; either strapped or riveted together. This armor protects the wearer's whole body. A fully-plated ensemble includes a matching aketon.
Armor Profile: 3 Defense, 4 Soak, 3 Hard Points, 5 Encumbrance; Fortify 4, Noisy 3. Price 5,950, Rarity 8.

NEW WEAPONS
Pavise
A pavise is essentially just an oversized shield which can be used as a portable wall. A character using a pavise can spend a maneuver to unequip and anchor it to a surface (or to disengage and equip it). While anchored, the pavise can't be used as a weapon, and doesn't occupy a hand, but characters who're engaged with the pavise can take cover behind it (providing 4 ranged defense).
Weapon Profile: Melee (Light), Damage (3 + Brawn), Critical 5, Range (Engaged), 2 Hard Points, 3 Encumbrance; Cumbersome 4, Defensive 2, Deflection 4, Fortify 2, Inaccurate 2, Knockdown. Price: 1,050, Rarity: 3.

Shield
A shield is a sturdy wooden board, covered with leather, and reinforced with a metal frame. They hang from the wearer's arm by a leather strap and are held onto by a metal handle. A shield is intended to be used to deflect blows and projectiles, but they also decent back-up weapons.
Weapon Profile: Melee (Light), Damage (2 + Brawn), Critical 5, Range (Engaged), 1 Hard Point, 2 Encumbrance; Cumbersome 2, Defensive 2, Deflection 2, Fortify 2, Inaccurate 2, Knockdown. Price: 350, Rarity: 2.

NEW ITEM DESIGN NOTES
My design goals diverge significantly from the GCRB guidelines with regard to the best available armor. In addition, while these items are based on historical research, they're not expected to be historically accurate. Amongst other things, they're created by the denizens of a magical world, so magic-use is assumed to be part of the crafting process.
First, typically only armor at least partially composed of a rigid material (such as metal or wood) provides any Defense.
Second, which armor types have Hard Points was determined by whether it made sense to be able to attach Spikes to it; though a high enough crafting check can still add Hard Point to a suit of armor without any (per the crarting rules in RoT).
Third, on the one hand I wanted the best suit of armor available to cap-out its wearer's Defense, but on the other I also wanted shields to remain viable throughout the campaign. Thus, the best armor provides 4 Defense if an Ironbound Rune is attached, meanwhile shields now provide a secondary defensive benefit which stacks with worn armor.
Finally, all of these items have been priced using the guidelines from the GCRB; with the exceptions of new qualities (which I've priced), and changes to Hard Point values (which I priced as "other qualities"). Where a range of values were given, I used the lowest price adjustment (meaning many of these items have room to grow more expensive). A few items (such as the Heavy Robe), are actually worth $0 due to lacking Hard Points, so I split the difference at $25.

About Aketons: Aketons are included (at no additional cost) in many of these suits of armor, and generally you'd have to wear the included Aketon in order to equip the rest of the suit of armor. However, they're still effectively two seperate suits of armor whose benefits don't stack; so if your maille ensemble is destroyed, you're still protected by the Aketon you wore beneath it.

Regarding Fortify: I invented this quality so that I would have an additional factor with which to distinguish various types of armor. I based it on the already extant talents and abilities which reduce the severity of Critical Injuries. The heaviest suits of armor grant a few ranks of Fortify, as do shields. Thankfully, multiple sources of fortify explicitly stack.

Regarding Noisy: It exists solely to replace the cludgy wording included in certain items used to impose the same penalty. The reason I've made it a quality is so that I can easily target it with other game elements. Such as a Talent that reduces the Noisy rating of worn armor by a few points, or a craftsmanship/attachment that modifies its Noisy rating.

I really like your Fortify quality! I may have to steal that. Thanks for sharing. 😃