A minor question regarding Mythos deck construction. It is in regards to Stage III of the deck.

It is clearly stated that while constructing Stage I & II you shuffle the individual stages (before adding to the final deck) but it doesn't say this about Stage III.

One thought is that since the other two are shuffled then the third should be as well but one player argues that since a shuffle isn't mentioned then the colours are added to the final deck in order (green then yellow) to make the third Stage progress from "easy" to "hard".

Any thoughts?

I think every stages are built in the same way: gather the number of cards listed on the GOO sheet, shuffle the stages individually and finally pile them up with II on the bottom, II in the middle and I on top.

Are you referring to the section on page 6 of the rulebook?

"During step 8 of setup, players build the Mythos deck.
This deck is built differently each game, based on the
instructions on the Ancient One sheet. To build this deck,
perform the following steps:
1. Separate all Mythos cards into three piles, based upon
the color on the front of the card (green, yellow, and
blue). Shuffle each separately, and place them facedown.

2. Build stage I of the Mythos deck by taking random
cards of the denoted colors and quantities (listed at the
bottom of the Ancient One sheet) and shuffling them
together. Do not look at the cards at this time.

3. Then build stage II of the Mythos deck (following the
same steps as stage I) and place it underthe stage I deck.

4. Finally, build stage III of the Mythos deck and place
it underthe stage II deck to create a single Mythos
deck. Once created, do not shuffle this deck. Place the
deck near the Ancient One sheet. Return all remaining
Mythos cards to the game box without looking at them."

In step (2) it tells you to "build stage I" and then explains how to build a stage. In step (3) it says to "build stage II" and refers you back to the instuctions in step (2) on how to build a stage. By step (3) they assume you understand the steps on how to build a stage and they are running out of space so they just say "build stage III" without explaining how to build a stage again.

So, they are telling you to shuffle stage III before adding it to the deck merely by saying "build stage III".

Hope that helps.

Mythos Deck Construction is one of my favorite mechanics in Eldritch Horror.