Penalty points remain valid for 'totting up' purposes for three years but they can't actually be removed from a licence until four years are up. (There are longer periods for some more serious motoring offences but it's 4 years for speeding).
Anyone acquiring 12 (or more points) during a 3-year period must appear before a court. The court is required to ban them from driving unless there are genuinely exceptional circumstances. (Courts are instructed that the possibility of a driver losing his job if he is banned from driving does NOT qualify as an 'exceptional circumstance'. Courts should normally only consider circumstances where others might suffer if a driver was to lose his licence, such as a parent being unable to take their seriously ill child to regular appointments at a cancer clinic).