The constant searching for 3G network drains the battery. When you're hooked to 3G, it only uses the 3G network and it doesn't need to search for any other network. When on 2G it has to maintain the 2G connection + search for 3G so it will drain more than with only 3G. Also with good/v. good 3G connection the 3G battery drain isn't a lot bigger than 2G battery drain.