For fun: here's the same figure, but this time instead of keeping the contact points on a circle of diameter 1 (with the idea of having a circular frame perimeter), we make the convex contact polygon have perimeter π (the perimeter of a circle of diameter 1). This way, the frame is no bigger than it needs to be to house the wheels, and gets the wheels as far apart as possible.
Edit: oops, my spreadsheet was sort of a mess, so there was an error in this calculation. I've updated the figure below.
Now that we are taking full advantage of the contact shape and getting the wheels further apart with the same frame perimeter, the 3 wheel configuration is actually better than the 4 wheel, with much lower spread between min and max, and much much higher minimum moment. (The max moment for 4 wheel is still the best, but is pretty unusual that anyone is ever tipping in that direction.) It actually gets worse as you approach a continuum of contact points all along a circle (n=infinity).
However, the circular (or near circular) frame would arguably be better than a triangle when you are worried about getting out of t-bones, pins, tight spots, etc., so the other figure should not be forgotten.