Or fairies just poof into existence instead of being birthed....
I think - for sanity's sake - we may want to go with this one...
But that's completely off-topic! (I think...) My guess is, because I've yet to read any of the Three Fairies manga or the translated dialogue from 12.8, this:
Star's likely the most reasonably intelligent one. Book knowledge and common sense, here.
Sunny's probably more along the lines of Cirno: overconfident as hell and likely to rush into things.
Luna... eh... I'm just gonna echo what other people've said. Not /stupid/, but not horribly bright and relies on her power too much. (because the sudden/complete lack of noise totally gives her away)
Edit: Basing my admittedly lame assumptions on what I've read about them on TVTropes and the wiki.