Well you already have the best system, so that's a good start. Out of the Wii and PS3, I would say the PS3 is better by far. BUT....I would say get the Wii. There's just not enough good exclusive games out there on the PS3 to warrant buying it if you already have a 360. The Wii is really fun, but not really for hardcore gamers. But again, since you already have the best of the 3 then the Wii is a good compliment to it if you just want to have something for parties and friends and such.
Having said all that, I own all 3 but highly regret buying the PS3 because I already had the 360. I mean there are some good exclusives for the PS3 (Metal Gear, LittleBigPlanet, Killzone 2), but most of the bigger games come out for both systems anyway.