I had a similar experience, even though I thought I had enough bricks in their shallow pool that they could all get out. They were about ten days old and had started popping in and out of that little pool like pros. (I had just moved a couple of the bricks out that morning to give them a little more room.) It was a soul destroying experience. I also had three when I found them that were just barely clinging to life. I took them into the laundry room and rubbed them dry with warm towels heated in the dryer and put them under a heat lamp (at a safe distance). And watched over them like a hawk for awhile and all three made it back (eventually) -- prayers for yours!. And though my momma duck seemed to still know that she had lost a few, she was greatly relieved to have the three back.
I have a friend that is the most protective, kind, concerned animal lover I know and she lost three ducklings to drowning. I think she still feels destroyed by it at times to this day. We hear all the warnings and can't imagine how easily this happens to these little buggers that seem so at home in the water.