What might be causing these cats' allergies?

I might get a second opinion from another vet. Scabbing and hair loss can be signs of infection - bacterial, fungal, parasites/mites.

It may not actually be allergies; it could be psychological/stress based. One of my cats has this and it is called something like "stress licking/stress grooming." One of my cats kept licking herself obsessively until she made the skin raw and scabby. I took her to the vet, hoping it would be an easily resolved allergy issue. Nope. She is not allergic to anything. It is a stress response. When she's stressed, she licks to soothe herself. This sets in motion a seemingly endless cycle: 1. Cat is stressed. 2. Cat grooms obsessively to self-soothe. 3. Abrasive cat tongue licks off fur and then makes skin raw. 4. Skin is sore and itchy. 5. Cat licks the sore spot in her tiny-brained attempt to make it feel better (and to self-soothe, because now she's even more stressed out). 6. Repeat ad nauseam. So the only thing I can do for her is apply an ointment called http://veterinarynews.dvm360.com/dvm/article/articleDetail.jsp?id=183812 to help heal the skin and break the cycle at point #4. He said I could try to find out what her stressors are and eliminate them, although with cats this is easier said than done. Since two of your mom's cats really dislike the other one, it may be making all three of them stressed. This might be causing the overgrooming commences, setting in motion the cycle I outlined above. Even if one is avoiding the other two, they can all smell each others' presence. Your mom could try temporarily rehoming the cat the other two don't like, and seeing if that solves the problem. (And if it does, then she might need to permanently rehome it.)

Have you tried changing to a new kitty litter? What do they use now? I knew someone whose cat was allergic to corn-based litter, but fragrances from clumping/deodorizing litter, wheat-based litter, or other compositions are also a possibility. Maybe switch to a newspaper-based litter (Yesterday's News) or unscented clay to see if that helps? The wheat-free food is a good move even if the food is not the allergy cause.

Check if any of the products you use in your house have something like "new formula" or "new concentration" or "new anything" on the labels? Also check with your neighbors to see if they're using any kind of lawn chemicals or fertilizer. In a hilarious plot twist maybe they have become allergic to humans?

Are there other potential avenues to explore here? The http://healthypets.mercola.com/sites/healthypets/archive/2099/11/30/flame-retardant-chemicals-affect-thyroid.aspx?

What litter do they use? If it's scented, too, that may cause an allergic reaction. Does anyone in the household smoke?

I'd look at every cleaning product used in the house, and switch over to white vinegar, toss the scented stuff, etc. They also eat regular grocery-store dry food supplemented with Fancy Feast. Maybe switch over to the high-grade stuff the vet sells?

A roommate of mine had a cat that was allergic to grain and when we switched her diet to pure tuna fish, she cleared right up. She had rings of missing fur around her tail, it was so heartbreaking, but the pure tuna diet really helped her. It's odd, though, that you'd have an unrelated cat with the same issue. Worth a try to change their diet. Just be careful to only try one change at a time, and give it some time to take effect, so that you know what you've done that works.

Has your mom used any kind of carpet powder? I visited a friend who had used this and my dog had a horrible allergic reaction. It was Arm & Hammer Pet Fresh.