Well first off, CALM DOWN!!!!!!!!!!!!!!!!!!
No one can tell you what to do or say. To be the GC broken record: BE YOURSELF. They've liked you so far, so don't change now. These are people that you are going to be spending a VERY large portion of your college career with.
DON'T's
don't act like a total idiot (i think you've got that one covered)
don't talk about other groups
don't talk about how you don't like the 4th house
don't try to plan out your conversations- have a couple questions in mind, yeah, but don't write a script
Let it flow!
DO's
be calm, cool, and collected (easier said than done)
HAVE FUN!!!!!!!!!! (majorly important)
make sure the house knows how much you're interested- but don't be like "this is the only house i want" etc. etc. Find the fuzzy medium
Just settle down- go take a walk, do something, anything to get your mind off of it. Don't stress and freak out, it's not going to help you