I'm going to guess Kat, Taylor, Elliott.I think they want an all-male finale (they've been carping on it all season), so they'll put Kat first -- and I think they're more likely to try to pimp Elliott than Taylor.
HOWEVER, the entertainment-based, non-pimp method of ordering a program is to start big & end big...so if Taylor's got a really good, really big number (Try a Little Tenderness?), he may go last.
I think Kat will be first regardless.