I'm just trying to figure out if 20x12 wheels and 35" tires would fit with a leveling kit and 3" body lift.

 

I'm going to go on a whim here and say no. I don't have a lot of experience with the big offsets like that, but I'm running a 5" lift with 35x12.5 tires on 9" wide wheels with -12 offset and I'm already pretty close to the bumper and the back of the fender.

I'd go to a tire shop, I bought my wheels at my local 4 wheel parts and they mounted my tires on the wheels before I purchased them to make sure I liked them and that they would clear. Maybe ask a tire shop if they'd do a test fit for you.