Time Machine w/ Multiple Computers

Ok sorry if this has been asked before, I am actually re-installing leopard as I was travelling the last 3 months and just did an upgrade, as opposed to an erase and install (thus creating problems like being unable to burn cd/dvds).

Anyway, I have an iMac G5 2.O ghz, a MBP C2D 2.33, and a MBP C2D 2.16. I know that apple got rid of time support action on the airport, but am I able to connect an external HD to my iMac (like 1TB) and have it back up all three computers? It seems pointless to carry around an external HD with a laptop.