With that selection you've made already (DL, US, AA), you've covered the three major airline alliances. At this point, adding programs is strictly going to be worse for you.

Why have 15k stuck in each of a Delta and Continental account, useless for redemption, when you could have 30k in a Delta account (which is usable for something)? Or at the scale you're talking, why have 50k in each, when you could have 100k in one, which is enough for some premium redemption?

Frankly, I'd be looking into consolidating down to even one program, just to keep those balances high. It sucks when you can get three premium seats, but on three separate planes and three separate carriers.