Wii is more of a party system, if you have friends around often who are also into gaming, this may be a viable option. It sucks for serious games, though. The online is complete rubbish, because they dont use usernames, instead each game has a friend code.
360 is epic, but it's main downfall is it's shelf life, read up on the whole "Ring of Death" plaguing it lately. The online is of the best quality, but for a price.
PS3 has a lot of games the 360 has and a lot of exclusives coming out in future. The price tag is steep because of the blue ray player, however it's one of the cheapest blue ray systems on the market. Online play is free.
It's your choice, they all have their pro's and cons. Best way to go about it is check out the games you'd like for each system, weigh up which system has the most games you like; currently and in future.