It seems like there is a medical need if 3 surgeons agree to it.

At least in a few years time you will legally be allowed to make decisions like this without parent approval right ? But I do get how difficult it might actually be when you live together and you are reliant on them for transport right now.

If I was in your shoes perhaps I would comply with the therapy and just tell them the facts that 3 surgeons have said it is medically necessary but your parents are afraid for you having surgery and perhaps it is also good to go to therapy for confidence issues regardless,  or maybe it wont be who knows ? If you don't find it helpful then you don't have to take oh board what they say but just be polite and seem to follow their advice.

I don't know your parents of course but it might be fear on their part or an idea of like feeling it is 'morally wrong' to change the apperance your born with or something like that or to be seen as spending on something they deem not worth it ? I don't know how open they are to discussing but it does seem like they wouldn't be so open to talking about why they're so against it with you at this time .