It is 2 out of 3 times, and he should ask at the end of part 3 (after bombing, in your case). Saving the spy counts, so, it you scared both sides with a fear potion in part 2, it should be enough.
Peace is possible with every choice btw, and , if you do, Thane joins the tavern regardless of if he asked here or not, it just changes where he is during the war.
I had no idea peace was possible no matter the choice. My thought process was: save the spy, say yes to the lizards but help Thane instead, then bomb the village. Which means i've helped Thane twice, however the option to invite him hasn't shown up. I just wanted to play through the version where Thane returns to the tavern instead of the bull village.
Just did some testing, it seems not helping Thane in part 3 prevents him from offering to move there, not that it matters, he is coming over in the neutral route regardless, but i am gonna add the detail to the guide.
Edit: it was actually already specified in the guide, but i made it more clear in various parts.
