You need to refer to the full offer on your UCAS portal and it will make it very clear as to whether a particular grade needs to be achieved in a particular subject.
my offer doesn’t specify which subject is needed at an A, therefore does it not matter which subject I get a B in?
The way your offer is worded, its 'AAB from these three subjects' - in any order. If the Uni wanted a specific subject/grade, it would be worded as something like 'AAB including A in Chemistry' etc.