This is from Eye for an Eye (3rd chapter). There is a wargor wearing a rusty chainmail (soak 3). Do I add 3 soak to the soak number in the Beastiary entry for a total of 6 soak? Or is this just a 'visual' representaion of the default stat (3 soak) that the Wargor has?

The general idea is that the soak in the monster stats represents what your typical monster will be soaking taking into account natural hide, armour, magic, and whatever else.

If the monster you're using is wearing unusual armour, etc, then you could add soak to show that. But istm, your typical Wargor, wearing bits and pieces of scrounged protection including rubbishy chain, will have a soak of 3.

So, what is his Soak value if the adventure states that he wears a chainmail? Is it still 3? or is it 3+3?

3

....................

In essence the soak value present on the Beastiary is there for GMs to have a value without needing to individually add gear to the enemies. If you do assign specific gear to an enemy, you'd typically replace the entry in the Beastiary with the new equipment's values.

For example, (and these are pure hypothetical numbers, since I don't have my books here) suppose an orc has a DR of 5 in the Beastiary entry. If you gave him a two-handed sword (DR7), you would use a DR of 7 for that orc instead of the 5 that is in the book.

We played the Wargor with the standard soak value, assuming that the Chainmail was included in the statistic line. This was probably a good choice, as we ended up with some pretty badly wounded teammates by the end of that fight.

That said, we did equip him with a Great Weapon by replacing the appropriate stats on the standard line.