I made a very similar post to this, but my idea was slightly different. Same concept though. I think they just need a minimum and maximum for each role.
DPS - 3 max / 1 min
Tank - 3 max / 2 min (some argued for 3 max / 2 min - either way really)
Healer - 2 max / 1 min
It would work. Don’t get hung up on the exact numbers, think about the concept. It can work.