the gcc 3 ebuild will not coexist with gcc 2.95 it seems. The fix is easy and will allow both compilers to exist on the system and allow 2.95 to be the default or visa-versa. A system like kde 2/3 can be used where both are prefixed with /usr/gcc/$(VERSION) and adding the default to the PATH.
It is not so easy, as the ./configure script that ships with gcc-3.0.x is broken, and do not generate the binaries correctly. Anyhow, the latest gcc-3.0.4-r3 will install parallel with gcc-2.95.3 if not during bootstrap. and works with most things if you: # export CC=gcc-3.0 # export CXx=g++-3.0