It’s going to come down to who is the best campaigner and who runs the best campaign from those three.
I can tell you right off, Palin is the best campaigner—Perry won’t play nationally and Romney is a dork.
I hope she runs the best campaign, too.
1) Perry and Bachmann team up, Perry choosing Bachmann as the running mate to steal some of the religious right / values voters (a mixed bag, as he would be 'reassuring' the Tea Party at the expense of many moderates and ALL Democrats)
2) Palin stumbles -- either a gaffe while under intense pressure: she has survived the barrage but has not been under a microscope in *quite* the same was as during the campaign. (As a private citizen, it was just Joe McGinniss outside her yard; as a candidate, it'll be half the "press corpse") OR a completely lousy choice for VP (I think with her background, she will NOT pick any of the likely choices for VP, just as she herself was a surprise). However, I must confess I literally cannot come up with a plausible dark-horse for her VP who will shore up her weakness on defense / foreign policy, while still being conservative or an unknown. (An unknown would give credibility to the cries of "gravitas" and "Caribou Barbie" in a way that an old hand would not.) The two most likely choices I can see would be Congressman West (still too green for the White House) or General Petraeus (military credibility up the yin-yang but don't know his campaigning skills or conservatism).
Cheers!