I've heard back from my friends at The Owl Foundation in Vineland, ON, Canada. These babies are younger than I thought. The tiniest one is now two days old, the next three days and the largest five days old. As short-eared owls can lay up to 10 eggs this is a small clutch. They also hatch as they were laid which accounts for the difference in size and age. Amazingly enough, most of the babies are fed enough so they all reach fledging age if all goes right. So far everyone here is doing just fine. The youngest is still weak but eating everything I give him.
Annick said that this species will imprint quickly so I'm using a short-eared owl mask the way I did with the baby golden eagle. Here it is. It worked wonderfully with TUMBLER, hopefully will do the same for these three wee ones.