Siege weapons: If you have three people operating it, can you reload in a single round?

Friends, a question about 5e siege weapons. For a ballista, the DMG says it takes one action to load, one action to aim, and one action to fire. If you have three people operating it, can you do that in a single round? Thankyou! Yes

— Mike Mearls (@mikemearls) December 8, 2018

Yes

— Mike Mearls (@mikemearls) December 8, 2018

Wouldn’t that mean it’s default crew size is 3? If you wan to fire once per round

— Mike Mearls (@mikemearls) December 8, 2018