Where is the 3rd hut supposed to go? I have reached my population quota and need the 3rd one built but my builders will not start. Where should I put them to get going?

What's the size of your population?

I guess i have not reached my quota, just my next milestone, as a new baby came along which brings me to 26. How many until I need to build the 3rd hut.

25 is a milestone, not a quota mark.... 30 is the quota for 2nd hut