does 3G use different towers or just different antennas/dishes/whatever on the same towers that are already there?
3G uses completely different equipment, which may or may not be mounted on the same towers. At least, if 3G = UMTS. EDGE (which is also a 3G technology) is just an update to GSM, so it uses the same towers. CDMA2000/EV-DO … I have no idea.
I usually have better signal strength with my new UMTS phone. However, I don't know whether that is due to a different location of cell towers, a better phone design (however, both my old GSM and my new UMTS phone were made by Nokia) or really a feature of UMTS.