spoilers?
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What are these three forest that show up for? I know Tehtir or whatever is how you get to sullendnesslar, but what are the other two forests for? I had a mini quest or two in each, but is that it?

Yep, that's it. The Small Teeth Pass wins the Most Boring Map Area in the Game, hands down. I can't see a single reason these map areas were restricted until Chapter 6, except to make you wait to get a really good Bow.

Then again, the bright side of having nearly unoccupied map areas is that if you're writing a MOD, you've got a good place to set some of the action.

I think Irenicus's dungeon is the most boring area of the game.

Which would be more boring--starting a game and being stuck in Jon's Dungeon for an hour (real-time), or going to the Small Teeth Pass and being stuck there for an hour?

I'd take the dungeon any day.