Or a 40' and two 20' would also be prototypical, usually with the 40' at one end, rather than in the middle. Or a 40' and a 20', with one at each end, and a space between.
The prototype, at least in the 60s and early 70s, often had three 20', one at each end, and one in the center. The problem is that that would take six "end" pedestals, so you'd have to steal a bunch from other cars.