I wouldn't do it because sometimes universities can be adamant that you should have completed all your A levels within the two-year period of sixth form.
Plus it wouldn't be much of a gap year if you're trying to learn 3 A levels in that one year.
Also, how would you even be able to include them on a university application? You wouldn't be able to get predicted grades, and so you'd have to wait for the second year after you finish sixth form when you actually have the grades to apply.