Failed gcse maths will university still accept?Watch
You can always do gcse maths at college when you've finished your a levels if they won't.
And no ICT degree will accept you with a 3.
You can resit it alongside your current qualifications - ask you school/college.
If they won't accept your grade 3, which I doubt they will, yes. I'm surprised you were able to do A levels without a grade 4.
If you can do it in school, definitely do that. Otherwise, you could just do a night class alongside school or resit next year.