It all depends on how strong your 3G signal is. I recently had to hang out in a hospital waiting room with very poor cell reception (0-1 bars). 3G was nominally available (3G icon instead of 1x) and I did get data (albeit slower than usual), but the battery drained from 80% to almost empty within 2 hours while I was reading email and browsing the web. In an area with reasonably reception, I can browse the web and do email all day and night, without draining the battery.
GPS always drains the battery pretty fast.