A reasonable work load, work split and proper management on the maids, of course, most important is the maid quality, is far more important. A big team of inefficient staff will only give you tons of trouble and chaos.
I have 2 helpers and 2 small kids. I don't think you need a third helper, as your older one is already 3 years old. By the time they turn 4 or 5, they can do many things on their own and will not need that much supervision at home. One of the maids should be able to keep the 2 kids company while the other maid cooks dinner.