When I went into sixth-form and did my A-Levels we HAD to do 4 for the first year, and drop down to 3 in our second year, if we did well enough. If we had been given the choice to do three or four then, personally, I definitely would've gone for three.
I'm by no means a very academic person, but I really struggled trying to do all the work for four different A-Levels.
I would say that doing three is your best bet, unless you really feel like you would be able to do four.
Good luck!!