You need 3 subjects above a U grade to move on to the a2 programme. If you only did 3 subjects, and got a U in one of that 3 then that may result in you not being able to do A2.
If I were you, think about another college, and ask them if you can do 2 A2's and 1 AS (my friend did this, it wasn't an issue as long as she was studying there full time) this way you'd have to spend 3 years at college.
Alternatively, if you're not happy with the other two AS grades that you got, and feel like you need a fresh start and get better grades, just start from AS again.
If no colleges are taking you on (which I doubt would happen) sit your exams externally at a centre. I would advise you to do this if you're an independent worker, likes to revise on their own and don't depend on teachers assistance. Or else, get a tutor!
I know how it feels to do badly, but you have time and opportunities that can help you make up for it!