Well with three I run into this sort of thing a lot since I don't necessarily have time for three classes a week, so someone is always not in class (if indeed anyone is in class...) I just make sure each dog gets dedicated one on one time in "different" environments each week. They don't really care whether it's class or not. They just want to get out, think, do, interact. So if one dog's in two classes and the other two are in none, that's how it work out. The other two get their time other times, other places. Or, in the case of a dog with an incredibly short attention span, they come along to class and I work them just a couple minutes before/after class, somewhere out of the way, during class changes.