Hi again Amy,
Twin A for me was head down the entire pregnancy and twin B chopped and changed positions a few times but I never felt it..I am not sure when the latest time is that they can change position..It is pretty squishy in there for them now..I remember my OB telling me at 27 weeks I was the size of a woman fullterm with one baby....so space is extremely limited..
I have been thinking about the car seat dilemma as well..We are not pregnant yet but I would probably in your case put Stella in the middle and the twins either side..
What sort of car do you have???DH and I were discussing we may need a van next time if we concieve twins again...LOL