Although an upgrade, vs. fresh install, is possible, it is not recommended, especially going from FC2 to FC3, as FC3 uses udev/hal and FC2 does not. There are tons of problems doing an upgrade. So, fresh install is the best way to go.
Either way, you'll need the 4 install iso disks..... no simple, one click rpm to do this.
If it were me, I would bypass FC3 and go right to FC4, as the bugs with Hal and udev seem to have been worked out with FC4. .... just my opinion.