You are correct. There are only three options available. T-Mobile, Verizon and AT&T (All post-paid providers as it should be). You are not going to find any pre-paid providers in that list. There is however, a fourth option, and that is the SIM-FREE version of the iPhone. This is the unlocked version of the iPhone. This version will work on MetroPCS. The thing is that if you purchase the SIM-FREE version then you will have to pay the Full Price of the iPhone up front. You will not get any discounts that may be offered with the iPhones associated with the other 3 carriers. What you will have instead is an unlocked iPhone which will function with every carrier in the US and Canada and most of the world.
Axel F.