No, @VickiSoCal, Oxbridge definitely is not interested in your having a variety of subjects (and fwiw, there are no ‘majors’- you study only what you sign up to study). Explicitly, tutors who do admissions say ‘we aren’t interested in how well rounded the student is- we want students who love our subject as much as we do’ (you can hear that on one of the Oxford admissions podcasts on iTunes, and I have heard it over and over again in person). Unless you are applying for physics & philosophy, the physics and math departments are all about math & physics.
Specifically, you must apply with physics and calc BC and they really like it if you can apply with a further math (which you can, though it’s informal as a classroom grade won’t officially count).
Oxford only requires 3 APs, and some colleges at Cambridge do as well (Kings, for example).
But: both colleges have their own test that you also have to take for math or physics.
Cambridge has the STEP paper, which is taken in June of your senior year. So, if you apply to Cambridge, get an interview and then get an offer it will be ‘conditional’ on passing the STEP paper with a specific score (see a sample [url=<a href=“http://www.admissionstestingservice.org/for-test-takers/step/preparing-for-step/%5Dhere%5B/here”>http://www.admissionstestingservice.org/for-test-takers/step/preparing-for-step/]here[/here].
For Oxford it will be the Physics or Math Aptitude Test (PAT or MAT) (depending on what you apply for), and they are offered in early November. If your overall application- test scores, Personal Statement, LoR and PAT/MAT score- is strong enough you are offered an interview; if after interview you are offered a place it will either be ‘uncondtional’ (ie, ‘you’re all set & we’ll send you a welcome packet in August with more info’) or ‘conditional’ on exams you are taking in June (imo, unlikely for you, as you have everything relevant).
I would suggest you read the descriptions of each course at each uni and see which is more interesting to you- you will find details right down to how your day is structured and what you take each term online.
And yes, imo, you are a credible candidate.