Well, I guess it depends on what your third A level is.
On an open day last year to see the chemistry department at Oxford, I actually spoke to a number of students there who had taken something like English language as a third A level.
Since you have an excellent set of GCSE grades and A level predictions, provided your UCAS application is good, you will almost certainly get an interview (Oxford interviews most of their chemistry applicants, I think usually about 90% of them). After which point, your interview performance will decide whether you get a place or not. Someone else with the same grade profile and a third STEM subject could blunder their way through the interviews and end up rejected all too easily.