There is a maintenance cost far beyond what just the Checks are. The cost of the checks are just that -- costs for the checks. And, outside of D-Checks, those costs tend to be fairly low compared to the actual upkeep costs. They don't include any other maintenance costs, no administrative costs, no costs to train your mechanics, etc, etc, etc.
Check your Fleet Commonality page to get a breakdown of your Maintenance costs outside of your checks.
Every plane you add contributes a decreasing amount to its overhead costs (so the 10th ATR costs less to maintain than the 1st) and each fleet type you add also adds to the costs of maintaining your existing fleets. So you get a nice benefit for adding more planes of the same model to your fleet and a hit for every new fleet type you add to your overall fleet.
So you're really getting hammered by having 4 planes of 3 different types.