You better be careful if you get a package with a crappy transfer limit like 1 GB and one of the people in your house turns out to be a heavier than average user :p Heck, even with regular users, you effectively only have 333 odd MB each, which is pretty tight if one of you suddenly decides to download a movie which could average at over twice that :p
EDIT: Also I definitely do not recommend a wireless router for 3+ devices, especially if you're gonna be sharing files amongst yourselves :p Not to mention the extra security implications and possibilities of people tapping in on your LAN and stealing your bandwidth :p Even with MAC protection enabled on the wireless router, they could possibly DOS you instead by scanning for your MAC addresses then faking that same address on their own equipment :D