Since she is so young, I would see if she would eat a small amount of Mare's Match pellets. Gradually increase her ration to the recommended amount on the container so you don't upset her belly. If she won't eat pellets yet (which she should be willing to eat some pellets at 3 months) you can mix up a bit of milk replacer and feed her that out of a bucket in addition to the pellets. You may also supplement with some adult horse feed or a feed for growing horses (whatever you would plan to feed her after her normal weaning), to get her used to the taste. Just mix a small amount in with the Mare's Match and gradually increase the ratio of adult feed to Mare's Match as she approaches 6 months. Make sure to have fresh water and quality forage available to her at all times, and a pasture buddy will help her stay emotionally sound.
I recommend the Mare's Match only because she is so young and this type of feed is nutritionally balanced for a horse that should still have access to mother's milk. She will have a better start if she gets a feed like this.
If you have any questions or concerns, you vet should be more than willing to help you out and give you more specific advice based on your horse and your situation.