There's a number of factors at play. Your personal statement will be a strong deciding factor too. Putting it down as your first choice helps as well. You may want to consider contacting whoever is in charge of the course you have applied for (or at the least the first point of contact for potential undergrads in relation to your course) by email and asking them about their usual leniency.
My personal experience is this:
I applied to Aston to study Psychology BSc (as my first choice uni) with predicted grades of BBC. The standard offer for Psychology was ABB. (Keep in mind I got CCC in year 12). When I went to the Aston Uni Psychology Open day, I asked a lecturer who was the first year course leader (something along those lines) about their typical leniency and was informed: BBB is fine. They would take a BBC (even if they only did 3 A levels, which I did), a BCC student would be accepted given that they did a fourth A level previously, but anything below, unlikely. I got a BCC in year 13 so I presume I must've had a decent personal statement or another factor such as how this was the first year (as far as I was aware) that uni's didn't have a cap on how many students they could take on.
Hope that helps, other than that, the summary advice by Horsewithnoname is pretty good. Past/specimen papers are usually the way to go.