Acraping should be done and any bacteria found should be tested for the best antibiotic for treating it.
A vet should take a sample of the infected area and observe the cells under a microscope to look for things like yeast, etc. Also there is a dye they can use and if it changes a certain color, then that leads to what the culprit is. So none of the three vets you went to did this, since you mentionrd no lab testing was done? They prescribed pills without lab testing being done? Lab testing is the best way to rule things out and find the underlying cause. If my vet didn't do lab testing when I presented a dog with this situation, I'd demand it or go elsewhere.
I have a French Bulldog myself and years ago a dermatology veterinary specialist said you can clean the folds of dogs like bulldogs with those round Clearasil acne pads you buy in the drugstore. Works great on both my dogs. Never a problem since using it. Just be careful that you don't get it too close to the eyes. Cleaning with water and a herbal shampoo won't do a thing if it is indeed a yeast infection.
What are you feeding this dog? You might try switching to a high quality grain-free food if you already are not feeding one. Many skin issues are caused by food allergies, especially cheap dog foods high in grains like wheat, corn and soy. Just thinking that of a possibility as this seems to be cyclical in nature. Or possibly even an environmental allergy, although these aren't as likely as other things.. If the food is the problem, you can clean, medicate all day but it will still happen if you keep giving the same food. Wouldn't hurt to try. Also, a 50/50 mixture of organic apple cider vinegar and water might help after you shampoo.. Great anti-fungal and anti-bacterial properties are in organic acv.