In my days with Laker, the charges were based on MLW and not MTOW. We had six DC-10-10s. Then we got five DC-10-30s (which had the centre leg and were much heavier).
The centre leg could be operated independently.
Fred came up with this great wheeze whereby he could save money on short routes by landing with the centre gear retracted.
Then he discovered that to get the centre leg back down again while the aeroplane was on the ground involved jacking the aircraft up. Alternatively, the simplest way was to take-off as a #10, put the centre leg down and land as a #30 but that was quite impractical so the wheeze did not last long!