My girlie had a foal at 4 (this was before i got her). I personally though that was a little young but I suppose breeders dont mind and I know some people will disagree with me
I put a 3yo in foal, she took AI on first attempt so I had minimum costs to get her in foal. She has bloomed through this winter. I don't think she is too young as she will be a 4yo when the foal arrives. She is a very healthy and well grown mare. and had already proved herself inhand winning young stock championships & a good BEF 1st Premium AND she has proven bloodlines.
She would not of been broken this winter anyway and as it is the foal will be a early one, so she will be broken at the end of summer when the foal is off her. She wills till be a 4yo when broken so will be produced slowly through next winter to be ready to start her career competitively as a 5yo.
we breed some at 2, so they have the foals when they are late 3 yr olds. we only do this if they are mature enough. I believe they have been doing it this was on the continent for years.