My first thought as well was, "Does your daughter want to do this?" Just the way that this has been phrased sounds to me like a parent wanting to provide some opportunities for the child, but the child may not actually be as interested as the parent.
Do you own these 3 year olds? Have you just started with horses? How old is your daughter? What is your horse experience? What kind of shows?
This sounds to me like a situation where you may not know the time and cost involved in doing this.
Personally, I don't show at all, but if I was going to the first thing I would be doing is working with a horse that knows more than I do, not a 3 year old. Next steps would be to find a trainer.
If it's just for fun, attend shows, ask questions, read, practice, ask for critiques (and be able to take them) and be sure the horseman/woman is matched to the proper horse. Not someone inexperienced with an equally inexperienced horse.