UCAS - If a university asks for ABC and you get BBB would they accept you?

They might do, they might not do. It is completely up to them and they probably wouldn't give you a concrete answer until results day! It will depend on the relevance of the subject you didn't do so well in and the number of spaces on the unis course. It isn't pointless applying but as you get 5 applications you can make it is worth applying to a couple that you think you are likely to get into (although only apply if you actually want to go) as well as some tougher ones.