Maybe I'm just dumb but I don't really understand why you would even want to breed a 2/3/4 year old?
Wouldn't the stallion you would be breeding to probably still be around when the mare is older? If you're worried about the stud fee, couldn't you just buy a breeding now and hold on to it until your mare is older (I think studs do that? Letting someone buy a breeding now for later?)...?
Breeding a 2/3/4 year old seems an awful lot like bringing one foal into the world just so it can create a bunch of other foals... That doesn't really make sense to me.
Also, side note, I have seen some fillies that looked gorgeous as youngsters but as they aged and filled out fully, it became apparent that they weren't as A+ as they looked at 2 or 3 years old.
Personally, if I were in the position to breed a mare, that mare (if we forget about the near perfect conformation aspect) would be completely broke, trained in a specific discipline, shown with excellent results (or, in the case of ranch horses and others like that, have been doing her job well and for a long time), and be fully sound even after 10 or more years of hard work (barring any non-conformation-related injuries picked up along the way).
You can't forecast with 100% certainty that a 3 year old filly is going to be an amazing horse in her discipline. You can assume from bloodlines and such but there's no way to tell for sure before she actually gets into training and showing.
Personally, I've seen enough A+ mares that ended up in nasty situations that I don't really see the need to be popping out "maybe's". I can see A+ mare plus A+ stallion as being a good, even great, thing but breeding an unproven mare to a proven [or unproven! O.o] stallion seems foolhardy.
Anyway, that's my little rant. Sorry!
ETA: Piaffe, I've heard horror stories of young mares getting too tired halfway through giving birth and ending up dying with their baby. However, I don't know anyone who's had that happen and I sure hope that wouldn't happen if there was any sort of mare supervision going on...
Another fact is that at the summer camp I work at, there was this little paint mare who was maybe 2, if that, and she was preggo. That entire summer she was unbelievably tired. She was basically like an old old old horse in the body of a young horse. I'm not sure if that was due to her pregnancy or the fact that we had to ride her anyway in her condition (hate that about this camp!!) but in any case it was sad. I'm not sure what happened to her or her baby since she didn't come back the next summer, but I can't imagine it being anything really great, knowing her owners. :(