How do I transfer music from one iTunes list to another?

Alright, so I’ve got like 3 different computers, and I have an iTunes list on each of them and have different music on all of them. So, is there a way that I can compile all my music onto one computer? I know there’s a website out there, but I can’t remember what it’s called. Other than burning all my music onto database CDs or separate music discs, can I get all my songs from one computer to another?
Thank ya!

IM the files then import them to iTunes.