By sittings do you mean years? Because technically you can have 4 sittings for A levels if you do January and June exams for 2 years.
Usually universities will take situations into consideration. A lot of people take a gap year and do resits in their gap year, which usually isn't a problem if you get alright grades. Also you can write to the university and explain to them if there are extenuating circumstances as to why you need to take your exams over 3 years as opposed to 2.