What could be causing sores on my cats backsides beside their tails/?

This is the third cat we have had to get a raw draining soar along side of the tail that will not heal. It drains all the time and at first looks like a bite from another animal because it is a large hole. But the hole just keeps getting bigger and starts to drain and the hair falls out and the cats just gets sicker sicker. Has anyone got any idea. The vet just gave antibotics and had no comment. The first cat died and the second one got so sick we had to have her put to sleep now the third cat has it and I dont want to lose her too. The three cats have had this occur in the last 14 months. Please help.