I have a year 13, have you thought about getting her to the GP and asking if propanolol might be appropriate? (In my experience they are much happier to prescribe this than anti-depressants). Propanolol is a beta-blocker which dials down all the physiological symptoms of stress. There are lots of other things you can do to relieve anxiety a bit - address sleep issues, take time out, cold water at end of shower on the back of the neck (that one is a Michael Mosley tip), and apparently ibuprofen can reduce anxiety (research has shown it is effective in animal studies). It all feels so all-or-nothing for the at 18, it’s such a problem. Your dd sounds like a high achiever, and sounds like she will do well at whatever she wants to do in life. She probably needs a massive amount of reassurance that it is not the end of the world if she doesn’t do as well as she would like in subject 3. (I suspect it could be something like physics, that’s a really horrible subject at A level - my eldest did it, and the exam questions are so complicated, obscure and bizarre, that you only need to get 50 per cent of the papers correct, to get an A).
Would she be willing to have a complete break tomorrow, to reset? Would it help her to hear about some contingency plans? If meeting her grades for a uni course is her main worry, it might be that the course would give her a place anyway, even if she misses one of the grades. (Places like Bristol last year were confirming places even when grades had dropped substantially).
As an aside, we have SEN in our family, and anxiety is very common in things like autism. Perfectionism, catastrophising, lack of flexibility, inability to cope with changes to routine (ie, being away from school) can all be part of autism. Inability to focus, needing someone to be there with you / to buddy up with can be signs of adhd. Might be something to keep an eye open n at this stage.