So, this varies a lot depending on the course/university.
In general, Oxbridge, and medicine/dentistry/vet med will require you to sit three A-levels in one examination session, and complete all A-levels attempted in a 2 year period. Other universities or courses may have preferences - I imagine many Law courses would probably prefer a similar approach due the academically rigorous nature of the degree, and many of the "top" universities are likely to prefer candidates who have done so.
This means they will accept and consider an A-level sat early - this tends to be Maths, for those taking Maths and Further Maths, as many schools have students take the former in year 12 and the latter in year 13, rather than concurrently. However they tend to frown upon, and may well reject an application due to, a candidate taking one A-level early, and then only continuing with two A-levels.
The reasoning behind this is that a student who takes on A-level a year early then gets to spend extra time and resources focusing on fewer subjects, putting them at an advantage over those who do not. For the particular courses noted above as well, they are extremely academically demanding and entail more work than a full set of A-levels is likely to represent. As a result, if a candidate doesn't demonstrate they can handle three A-levels, how can the course providers know they'll be able to cope with the course itself?
Additionally, beyond the above, native language A-levels are rarely accepted by "top" universities as part of a standard 3 A-level offer, and normally don't form part of the offer unless it's specifically related to the course (such as languages degrees). You would be best advised in any case to take an additional third A-level this year, as you cover all your bases. You may find your options for applying considerably more limited if you do not.