Outside of Biology and Chemistry it doesn't matter what A level you pick providing that you make some sort of effort to study fairly well respected subject- so avoiding things like Science in Society, Culture and Communication etc (although I've seen people on this forum with offers with their third subject as business studies, PE etc so tbh I don't even know if it's THAT important you take traditionally "hard" A Levels.
So really, there's no "better subject" out of Maths, Physics or Psychology. I'd personally avoid forensics though, it's not a popular choice for medicine. The other three are probably the three most popular 3rd and 4th A levels. So just pick the ones you would enjoy and do well at. Getting the A is far more imoortant