Should/How to a reject my offers and apply to Cam

I just picked up my results which were 3 A*s, so now have a confirmed offer to do law at Southampton next year. I now feel that I should try and apply to Cambridge, and keep southampton as my insurance choice, but how do I do this? My subjects were Maths, Chemistry and Law. Is this enough I get into Cambridge? Would southampton still accept me if Cambridge didn't? Help! Also, I'm eligible for a £1000 per year per A* nursery