The GPS unit has nothing to do with the network connection (unless the GPS signal is not available, in which case the GPS may try to use the wifi hotspot or the cell your device is connected to in order to infer your location). It works independently of it if the GPS signal is available, and the network is only used to actually show the map. From the code snipped you posted, you only show the map when you reach an horizontal accuracy less than 100 meters, otherwise you let the GPS unit updating the location. However, if you try your code in the exact SAME place, the GPS unit on your device should always return the same updates. Therefore, I really do not understand the behaviour you are describing: this would be possible only if the GPS unit returned different updates for the same place when a different network connection is available because the GPS signal is not available. Are you seeing the same latitude/longitude in the two cases or not? Are you seeing the same accuracy? Be careful to measure this in exactly the same place in both cases.
If you obtain the same updates, then it may be possible that your 3G cellular connection is simply not powerful enough or is only apparently available, so that you did not get the map. Try testing the speed of your 3G network from the same place.
A related consideration. You should allow the GPS unit to work until either a specified amount of time elapses (say 20 seconds, use NSTimer for this) or until you reach a specified level of accuracy, whichever happens first. Otherwise you may end up consuming too much the battery.