It would be unconventional but it should be OK. She should however be furthering her application to Medicine throughout the gap years. I know next to nothing about student finance of non-EU nationals, but I'd check the student finance rules and give them a good read to see when exactly she'll be eligible for the tuition and maintenance loan.
Edit (misclicked "post"!): she can do more or less as she pleases on her gap years as long as she doesn't waste them. She doesn't necessarily have to study although some evidence of recent study will be welcomed by universities considering that it will have been 3 years since the end of school and the start of university. She can work, ideally in the healthcare sector, and try to find work experience in hospitals. If possible, however, I'd try to find a way to avoid your daughter taking 3 gap years as it seems a bit excessive and will probably play on her mind a bit. Can she study in another country where she might not have to pay as much?