Even with a 3" lift, you'll still see rubbing with 305's if you don't do some trimming on the bumper, 315's will probably be even more of a hassle. Besides, once you start getting tires that big, you'll need to start thinking about a regear. 285's fit no problem, 295's rub a little, but a spacer can probably fix that.
If you're looking for tires on the cheap, check out treadwright.com - I've heard a lot of good things about them, and will be ordering a set soon.