This is a good question!
Quick answer -yes.
Explanation - If you have a heal incantation spell that can heal multiple targets. For each non-mage creature that heals 4 or more damage, the Paladin will gain 1 valor. So in your example, as long as the 3 non-mage creatures have 4 or more damage and heal 4 or more damage from your healing incantation, you will gain 3 Valor.