Easy version:
Boot from fc3 install cd, choose upgrade, choose developement (or whatever comes with gcc installed default... probably not server, not desktop workstation... whatever the other one is).
Slightly harder yet still easy version:
Boot from fc3 install cd, choose upgrade, choose custom, find gcc package.
Slightly harder version (I'm trying to do this with a friend's laptop... it's actually proving rather difficult)
Find a binary version of gcc (you need a LOT of dependencies... glibc-devel, glibc, kernel-headers... and kernel-headers requires gcc IIRC... this is where I'm stuck at right now), and install the binary version. Meh.
Oh, and you might want to check out gcc 4.0.0