Welcome to the forum, gypsygirl525.
As others have said, most suggest not breeding Minis at all right now, since there are literally thousands, needing homes. Many are being sold in public kill-buyer auctions and fetching $2 each. Yes, you read that correctly. Well bred Minis with some championships under their belts are also not finding homes.
Who is your Mini? What is her pedigree? Is she registered? Is she as close to conformationally perfect as one could desire? What are her faults? Have you evaluated her faults honestly? Would you be willing to share pictures of her here and maybe the stallions you have in mind?
Are you absolutely and thoroughly aquainted with Minis and their known problems when giving birth? If not you should be. Several extremely well known and knowledgeable breeders last year, lost both mare and foal, during foaling. Breeding Minis is not for the faint of heart.
Certainly a Mini could be bred at three years old. The questions is, should she be bred at all? If you are in the US, then I'd suggest not breeding. There are many superb Minis available at reasonable prices.