Foundations of Stone quest card side 1B states: "When revealed: Reveal 1 card from the encounter deck per player, and add it to the staging area." In a 3 player game I would add three cards as part of the setup.

Am I correct that I would add an additional three cards to the staging area after players commit to questing giving us a potential total of six cards in the staging area to start?

Yes, you are correct.

You add 3 from the When revealed effect on 1b during setup, resolve those cards, then during quest phase you reveal another 3 cards and resolve them.

You even can have more than 6 cards if there are surge (don't remember if there are in Foundations of Stone)

Thanks