Since the Mono Project hasn't released any repository for FC3, I spend the last weeks trying to find a way to install them. I almost gave up on it untill I discovered
http://xplib.de/mono.php.
Just download all the RPMS to a new dir (I named it monorpms)
# su
# cd monorpms
# rpm -ivh *.rpm
If for any reason monodoc give's a problem, download it source rpm
# su
# rpmbuild --rebuild monodoc-1.0.4-2.fc3.mh.src.rpm
# rpm -ivh /usr/src/redhat/RPMS/i386/monodoc-1.0.4-2.fc3.mh.i386.rpm