Scab is not life threatening to a tree unless it is really bad, for instance if you live in a very rainy area, and have susceptible varieties. Sunset is good as it resists scab a lot, your other two are only slightly susceptible.
When leaves are wet for any length of time, and also if a tree is feeling weak for whatever reason, scab is worse. MM106 is a vigorous variety and I wonder if it best suited to container growing, if you have access to soil for planting it would be happier, otherwise you have to do a lot of watering and feeding, or re-potting. Try spreading some extra multipurpose compost on top of the container for now.