I didn't expect it to be this hard for everyone... XD For me this is quite easy:
Date: Yozora - I very much dislike her personality, and while she's hot, the other two are hotter.
Sex: Kobato - AFTER she grows up for a few more years. I have no interest in her as a loli, but a few years later...
Marry: Sena - I was a fan of hers even before I knew there was going to be an anime. No, I haven't read the novels or the manga, but I
have read some very nice doujins.
Seriously, though, Sena and Yozora both need some personality development before I'd be willing to have anything to do with them. Kobato seems alright personality-wise so far, though a bit spoiled.