Both 3 wheels frames I owned are flat, Powerslide Trinity 3x100 and 3x125. Wear and tear of the wheels on my right foot wears out more than my left due to my dominant dragging leg for T stop. As for the wear and tear of individual wheels for both feet, it seems to be wearing out at the same rate.
Some skaters tend to do T stop dragging the wheel at the toe more, whereas for me I dragged it flat. Due to this I just do a mirror swap of my wheels to balance out the wear and tear.