the third choice would be nice if it had an actual videocard with it instead of onboard graphics. the fact that it's a quad core is going to be more helpful than the other two pc's imo but for the two amd options both are upgradeable more so than the intel one.
at least the am3 socket is being used for current top amd cpu's where as intel's lga 775 socket isn't being used for new cpu's.
it's going to be harder to find upgrades for that over the amd but if you don't plan on upgrading the computer at any time in the future then the intel one would be slightly better for price.