Have you tried companion animals?
I would also suggest another horse or a companion animal. The weaning process for mare/foal can be rather traumatic if done very suddenly and cut and dried, like it sounds like your case. Also, at age 3 years, the foal is way overdue for weaning. Usually the weaning happens at 6 months or so. All that extra time has made the bond stronger for both mare and foal. This would explain part of the stress of separation at this point. Often times, it is best to make sure the mare and foal are already in a group of other mares/foals,and when separated, the foal will continue with the other foals, and the mare will remain with the herd she knows, lessening the separation anxiety (although it is sure to happen). At this point, the horses together will always have a strong bond, especially if they are the only horses in the pasture, and will have continuing anxiety at any kind of separation. I would consult with your Vet about this to get his/her opinion on the behavior factors of this particluar mare/foal and how best to deal with the situation.