There is no penalty commonality cost between 1 and 3, so what you saved is just the normal commonality a particular fleetgroup creates.

There is a penalty between 1 & 3, the cost is roughly doubled for each fleet. If a fleet of 20 737s costs you 1 million/month as your only planes, it will cost you about ~2 million/month for just the 20 737s if you also have 2 other fleets. It's quite easy for anybody to verify this just by clearing some schedules.

The big difference between 3 & 4 fleets is the penalty remains a fixed % at 3, but the % increases with fleetsize at 4. If you have 3 fleets of 200 planes each, each fleet will still only cost double what it would if they were your only 200 planes. But if you have 4 fleets of 150 planes each, each fleet will cost maybe 6-10 times what it would if they were your only 150 planes. Increase that to 4 groups of 200, and now they cost maybe 8-12 times as much, so the person with 4 x 200 pays 4-6 times as much for the same 600 planes as the person with 3 x 200 does.