Would I have to resit all my subjects if reapplying for medicine?

I got AAB and needed AAA; do I sit all 3 or just the one I got a B in?

Should I resit one of my A subjects and see if I can get an A*?

You need to check the university's policy on resits. AFAIK some (or all) medical schools require your A-Levels to be obtained in one whole sitting

https://www.themedicportal.com/medical-school-a-level-resit-policies/

you didn't answer my question - would I have to retake all 3 subjects or just the one in which I got a B

Some medical schools do allow resits, although some of them require you to get A*AA or higher on the second sitting

Have a look here for a table of resit policies for each med school
https://www.thestudentroom.co.uk/university/courses/medicine/medical-school-resit-policies

For those that do allow resits, have a further look at their websites for complete information and contact them if you are still not sure whether you would need to resit just the B or all of them
I think this is the best starting point for you and is definitely worth a try, if this fails you still have the option of grad med although this is more expensive and takes longer. I'm not sure if there are any foundation courses that would fit your circumstances, most are designed either for people with good grades but in the wrong subjects or for people from deprived areas. Check their entry requirements carefully if you are thinking about this route, but I would try resitting first.

thanks man,
I've emailed all 7 that allow resits, about this

I'm thinking of resitting all of them - I think I can get A*A*A this time - maybe it would increase my chances?