A level outside sixth form

Hey! So I do three a levels in my sixth form, two of which I’m quite confident about, but I’m failing my third one-and VERY badly. The problem is, there is no way to rectify this since my teacher doesn’t come to lessons (which is why I’m failing) and the school won’t do anything about it. I fear that the highest grade I can attain in this subject is a D probably. (For now) I am thinking of doing an extra a level at a place called London PT college. (No teaching just the exam) Because I really want at least A*A*A. If I do this, and end up getting an A in the subject I do outside my sixth form, but my third subject in the sixth form is a fail, will I still get into the uni that gave me offers? Since I hit the grade requirement but with the extra a level I do? Sorry if this was confusing I’m not very good with words.
PS: the third subject is not conditional towards my offers, meaning that I need to get the grade, but it doesn’t have to be that subject, but I have already submitted my ucas which states I’m doing that subject, and I intend to do it, but I’m just thinking whether I should do an extra one

Might be a silly question but whats stopping you from studying the subject youre failing outside of school instead?

I'd email or phone the universities' admissions teams directly to ask and to inform them of the situation. Even if you didn't need a specific grade in that subject, the subject itself may have played a part in the decision to offer you a place. But I think as long as the unis agree then you won't have to change anything on ucas. I feel like this could work as most unis only look at 3 A-levels, like if a uni doesn't consider the epq they won't care if you fail it but I really don't know. Good luck and I'm sorry your school isn't doing anything about the situation, that's genuinely insane.