Staying at 6th form for three years

If I stayed at 6th form for 3 years instead of 2, would it affect me getting into University?

Nope. Most people don't do so good in their first year of 6th form/college and end up retaking a year. As long as they still had good grades by the end of Alevels and a killer personal statement, they managed to get to uni. You'll be alright

Well I am currently in my second year of Alevels, but Chemistry isn't going my way at all. I was hoping my school would allow me to stay for another year and do one Alevel (e.g English) but was scared it would effect how Universities see me, so thank you

Naa you'll be okay. So did you wanna pick up English as an AS level then? I say unless you really wanna do it, don't bother wasting a whole year only doing only one subject. Focus on boosting your understanding of Chem if you're worried about ums points. If you believe you can achieve, you will. I did Chemistry in ALevels so if you're stuck on anything or have any questions, feel free to ask.
I mean I may have forgotten bits of I think but I still have my notes to look back on
Also, there's loads of chem threads around on TSR so look out for those.