Not necessarily. An offer of BBB could be construed by the uni as being "three A-levels offered, with no grade less than a B" - which ABC does not meet. University admissions staff don't "average" A-levels like that in general, unlike what the above suggests. They could reject you on results day if you got ABC, or AAC while your offer was BBB; whether they would is another question entirely, but it would be up to the uni entirely if they wanted to take you with those grades or not, whereas if you got BBB (or ABB, or AAB, etc, etc) they would be required to take you.