3 is what most cob breeders used when I had horses. They have the foal in their 4th summer.
But as has been said, there are so many going for nothing, or a couple of hundred pounds, my mum was given a beautiful section c mare last month. Plan to keep the baby for at least five years, till its riding well. Work out how much it'll cost over that time, including plenty of shoeing (even if it doesn't need shoes, people want a horse that's experienced in everything.) add 20 percent contingency and £600 emergency vet bills, for the five years. I know it's a hobby, but horses are a hole in the ground when it comes to cash and its worth being realistic before you commit to a project on this scale, but best of luck to you, I'm sure you'll put more time and care into it than many breeders do.