At 3, my son was swimming quite well without assistance. Breathing, proper arm strokes, diving, cannon balls, etc. The summer after his 3rd birthday, so he was 3.5, he was allowed to swim alone in the pool with an adult watching. He has never taken formal lessons, just backyard learning and watching others.
I would think your kids would benefit from group lessons. The mob mentality takes over and sometimes forces the child to learn. Plus, they'll get some additional socialization. Just IMO, put them in group lessons. I'm sure they'll love it!
From my iPhone, probably while BF'ing.