I bred one of my old mares at three, though she did conceive she soon reabsorbed the embryo. This was not necessarily anything to do with her age though.
When I worked at a stud, we had an accident where one of the young colts broke out of his yard, bashing through 2 fences and jumping the last to arrive in the filly paddock, where he met a lovely 2yr old filly......
Apparently though he and his member were cut to ribbons, there was still enough enthusiasm to do the job!
as 11 months and one very confuse stud mistress later (she didn't think he could have done the job) there, in the breaker barn, was a lovely foal.
The filly was a mum at only 3.
Though she and foalie were all healthy, we feel it stunted the filliy's growth some what, as while her full sister was a comfy 16.2, she barely made 15h. She was a warmblood
I do not think that it is a physical can she, question. More of a morally, should she. For myself, if I have a young broodie, I'm leaving them till 4 to breed just to give them their own time to mature.
Sorry about the spiel