Presuming you’re talking about uni admissions:
Have you already got those 3 A*s at A level i.e. they’re achieved grades, not predicted? If so, then your GCSEs will hardly matter at all, since they’ve got proof of your ability to do well in exams (they use GCSEs a bit when you have predicted grades because GCSEs are the only evidence they have of your actual exam performance).
If they are predicted grades, your GCSEs definitely aren’t the be-all and end-all so no your changes aren’t ruined, there’s no harm in trying this year and you’re very likely to get very good offers, but even if you don’t (which I doubt), you can always apply again next year with your achieved grades, and then your GCSEs won’t matter at all.