When can the kids bathe themselves?

I have been giving baths to these kids for five years now and I gotta tell you I am done. When we first started giving them baths they could fit in a little plastic tub that you would put on the countertop. Then as they got bigger we moved them into the bathtub. As time progressed they just kept getting bigger and more willful. Up until about a year and a half ago we were still bathing all three at the same time in the tub but then they got to big. Now we do it one at a time, and let me tell you I spend half the morning washing children. Iris is great she really loves a good soak and will float in the water for as long as we let her. As for the boys I have seriously thought about throwing their laundry in the tub with them due to the amount of agitation they generate. If we could somehow harness the energy of children I believe that we could solve the world's energy crisis. However my question is when do kids take on that responsibility on? When will they want to bathe alone?