Hey, there.
Firstly, unless you're considering Oxford, Cambridge, or Medicine, GCSEs are only a tick box; don't worry about them, just focus on your AS Level, and get those results as strong as possible.
You have English, Maths and a Science, so don't worry. And, if you get those results, no University, other than the ones mentioned, will mind.
Also, if you only sat that many, you won't be necessarily held to blame: the university will be aware that some schools don't sit as many, and probably won't hold you accountable.
Hope that helps, and welcome to the forum!