Well the thing with 3G is that if you don't have your data on (example if you are at home on wifi connection and your data is off), there is NO reason whatsoever to have 3G on. 3G has to do with having faster internet speeds on your cell phone (via data) , and being off does not affect signal or calls.
So its a waste to have it on all the time, because if you are at an area that doesn't have 3G coverage, having the 3G constantly on makes it search for 3G signal and thus draining battery.
What Cell service do you have?