Absolutely keep feeding if she will take it.

Reptiles in general have very slow metabolisms three doses are unlikely to have any effect at all. The last set of antibiotics I gave to a snake for RI (one of many rescues) was given over 6 weeks every 3 days there was a marked improvement after the first 25 days. In my experience this is quite typical. It is unfortunate that he guessed at which antibiotic would work a culture is much more efficient. Sadly after antibiotics have been given cultures are inconclusive until months after the last dose. It sounds as if your vet is extrapolating from treating cats.