Don't fill a dent that deep with bondo. It will crack, and probably fall out one day while you're trucking down the road.
Take some time with a body hammer and dolly and pound the dents out as much as possible. When you get the surface smooth enough that 1/8 inch of bondo will do the job you're there. In any case don't go over 1/4 inch thick with bondo, it's just not made for filling such deep depressions.