Are you sure you chose the 3G option?
Your phone will most likely have two options as shown in the following screenshot.
Cellular Data (shown as the British English equivalent "Mobile Data" here)
The first option will disable the 3G network and only enable your carrier's 2G network (which may provide GPRS, EDGE or 1xRTT data). If there's no 2G signal or the network doesn't support data then you wont be able to use it in the way that you want to.
What do you see in the top left of the phone? Your phone will show a different symbol for each type of network:
I mainly want to do this to save battery life. Any help is greatly appreciated. Thank you
What version of iOS do you have?Settings > General > Network > 3G on/off.
What version of iOS do you have?
If you have iOS 5 then the option to turn off 3G "only: is not currently available. It is floating around in the new Beta of iOS 5.0.3 so expect it to return as it does help with battery usage in some situations.
isnt it the iOS 5.1 beta 3 already???
Oh you can't really turn off 3G on Verizon/Sprint phones.
The phone uses 3G network for data and 1x network for voice. So if you disable cellular data, you're basically disabling 3G. You can't use 1x for data though.