No way any of the trio will die before the end of the series! (Though all bets are off as to who survives the last book!)

I'd vote for Percy (because he's so very annoying!), but I really think it will be someone new. JKR seems to create new characters only to eliminate them; Quirrel, that blond DADA teacher, Dolores Umbridge, Sirius... the list goes on and on. We hadn't heard of Cedric Diggory until book 4, and then he gets killed. So I don't think it will be anyone we have a strong attachment to yet.

But, of course, only JKR knows...