How good is your vet?
The bacterial strains that cause RIs in snakes are flora that are already present in the snake's system. In other words, if the vet is less than familiar with snakes, he may be targeting normal bacterial flora. Plus, all your snakes have this - correct?
After three rounds of antibiotics - all of which were selected due to cultures - I would suspect something other than bacterial.
Again, how good is your vet?