Try buying them first, it's much more legal. Plus it's nice to support those that pour their lives into bringing these games to a platform that is not know for it's gaming. Typically this encourages them to bring other games to the Mac platform, which is a Good Thing!
Doom 3 doesn't exist for the Mac yet.
Doom 2 is pretty old, it was originally for Mac OS 7.1 and above I believe. This means it won't run natively in OSX. It might run under Classic. It might run booted into OS9.