Applying for US grad schools with a B.A. from Cambridge?

Hi, I'm an international student planning to read Natural Sciences (Physical) at Cambridge this fall, and I'll probably choose the physics track within the Tripos to embark on a career as a physicist. Cambridge is great in physics but is sadly quite expensive, and I can only afford three years of education there. Just wondering whether it is easy for someone holding a B.A. from Cambridge to skip the fourth-year MPhil and apply for physics grad schools directly?

Thanks in advance if anyone could help with my questions!

If you mean skipping the MPhys not MPhil then you should be able to apply.

I'm a 3rd year Chemist at Cambridge and for what it's worth, I know one person (near the top of the year) who's got into a top US grad school for a PhD (think HYP, MIT, Stanford...). However, the vast majority of people continue to do the MSci before pursuing a doctorate, as especially in the UK for physical sciences a four year masters is a standard undergraduate course. You can by all means apply for grad school while in your 3rd year and see what you get before contemplating the financial implications of a fourth year, but certainly don't bank on it.

would you suggest this for all degrees?

It depends on the subject, but for sciences where most UK students will be doing a four year master's (most arts and social sciences are 3 year bachelor degrees), doing the master's can only help, short of spectacularly screwing it up, which you shouldn't if you made it that far. Bear in mind also that all American graduates will spend four years doing their bachelor's, so it might seem a little odd applying for courses right after your bachelor's, but it definitely happens and is sometimes successful. Like I said, just be prepared with other options if it doesn't work out (kinda applies to anything though tbh!).

what about if apply someone like Harvard or Caltech where you can only enter the graduate degree at a doctorate?

It's quite normal at the majority of US grad schools to go straight from BA to PhD. The issue here would be whether they Physics BA is viewed as a 'full' undergraduate degree since it is usual for students to leave with the MSc. I would do some preliminary checks with potential US universities. I'd check with the relevant department themselves (probably the PhD program director, or the Graduate Admissions coordinator, if they have one), as they are the gatekeepers to their programs.

Also, a way to infer the info from elsewhere by yourself might be to scour both the personnel pages of department websites, which usually list grad student's previous degrees, and academia.edu ( there's also a more science-centric version of this but I can't remember what it is called), where you can often get the same info. I imagine there'll be more than a couple of Cambridge grads at the top departments. Much less reliable, of course, but it will give you an idea of the range of qualifications incoming students have.