For your 3rd group, there should be no penalty added to the other groups, just the cost of the new group. Once you hit 30 aircraft or so in a fleet type, the incremental cost of adding another aircraft to any of the fleet types should be roughly the same. The first big penalty is from 3 types to 4 types, 4 to 5 is marginal, then 5 to 6 is another 100% jump. Each added type after that adds to the pain.
But short answer is types 1 to 3 should all cost you the minimum amount per frame, no penalty added.