1 in 12 got an A* so 3 A Stars can be found by binomial theorem:
[(4!/(1!3!))][(1/12)^3](11/12) if the probability is constant giving 11 in every 5184 students (0.2% of candidates). If 800000 people (made up number) sat A Levels in 2010, then 1698 students attained 3 A*s. On top of this, 1 in every 20736 students would have attained 4 A*s (39 Students); so i'd estimate 1737 students would have got 3 or more A*s in Britain.
Assumptions i've made:
everyone does 4 A Levels (which they don't)
probability is constant between each subject (not true- 29.9% of further maths students got an A* whilst 1.something % of media studies students did)
Each subject is independent of each other (not true- maths and further maths relate)
You either get an A* or you don't
800 000 people sat A levels