Food doesn’t cause UTI’s but depending on what is fed it can change urine volumes. High protein/salt foods can increase urine volume.
My Lab had a UTI when young. Like you I noted increased urination as the primary sign. But because increased urination can be caused by several things my vet insisted that she not have any antibiotics until a sample was taken directly from her bladder and cultured.
It concerns me that your dog has been on three medications and none seem to be working. If you didn’t have a culture to verify that bacteria were present and the cause of the symptoms, the reason they may not be working could be because there was no infection causing the signs. Or another possibility is that stones are present, as uncommon as that is in such a young dog.
