It is still a mystery to me if the F3 has both GSM and CDMA chips inside. If it did usually they'd advertise them as a "World Phone." I have my doubts, though. However, MetroPCS is selling the GSM Optimus F3 and the F6 for $99 each. I would get the F6 if I had to choose between the two, which I did to use on T-mobile. You'll still need to unlock them, which I also did with mine. I also flashed it with T-mobile firmware.
Good luck.