From what I know, 3A* will give you a chance at either. People are saying that you don't have a fantastic chance, which is true but don't take it personally. Basically no one has a fantastic chance for Oxford or Cambridge, so don't be put off by it. I'd apply if I were you, not really got anything to lose by applying to whichever you prefer (remember through UCAS you can apply for up to 5 unis but not both Oxford and Cambridge, so you could just put one of them down and still apply to places like Imperial).