How fast will data transfer rates be on an Iphone over a 3G network?

Unfortunately, this is a tough question to answer. The 3G protocol does not have a defined or guaranteed speed. The International Telecommunication Union estimated that the speed fo the 3G network would be somewhere between 384kbits per second if you were moving to a maximum of 2Mbits/second if you were stationary.

Many advertisements claim that 3G will provide "broadband" speeds to your phone. This is true, though the data transfer you will likely be slower than your home internet connection.

Actual transmission rates will depend on your location, your speed of movement, the coverage of your respective carrier, and multiple other factors.