I'm not sure if these subjects are the required ones to study Medicine.

Psychology though isn't regarded as being one of the sciences though and will be held in lower esteem than say Physics.

I guess so, thank-you.

As long as you have chemistry and biology most unis (all, except Cambridge) will not care whether you have done psychology or physics. The most important thing is to get the grades, if you believe you have a better chance of getting an A in psychology then do that, especially if there is any chance of you struggling in physics - how did you find physics and maths at GCSE.

The most important thing is that you get your 3 As, if you get AAA in chemistry, biology and psychology then you have every chance of getting into medicine, but if you get AAB in chemistry, biology and physics then you would have very little chance. As you have suggested that you would find physics difficult, then psychology may be the better choice for you.