Make sure you have a C2D Mac mini before you get 3 GB RAM. If you have a CD or CS Mac mini 2GB is the maximum you can install.
I don't think the genius bar will do a third-party RAM installation for you. But a decent AASP should. Check what AASPs there are in your area and give them a call. If they do it, although the RAM won't be covered by your Mac's warranty, the rest of the Mac will still be fully covered.