Bad Piggies Rise and Swine Level 2-21 can be 3-starred using a single contraption. A two frame cart with a soda bottle on each side will do the trick. Trigger the right one to collect the star, then the second to cross the finish line.
If you have a different strategy for completing this level feel free to leave a comment below.
My cart kept tipping over as it came back down the hill on the left. Moved the pig into the left square and got it first try.