Bad Piggies Flight in the Night Level 4-21 can be three starred using a single contraption. Use a straight-line 4-box cart with two engines, two upward-facing propellers, a left-facing propeller, and a small wheel in the top-left corner. Use the propellers to travel along the roof of the level, grabbing the star box and avoiding the TNT along your journey.
If you have a different strategy for completing this level feel free to leave a comment below.
I had the other wheel on the far right also and clear this with one try
I also think that the contraption is much easier to control if you have the other mini-wheel on the top-right. It will then roll across the turf, rather than you having to fly it manually, which causes some problems, for me, at least;)