Help with choosing a third a-level to go with psychology and biology?

Hello
When I'm older I currently want to be a clinical psychologist, or at least something similar. I've so far chosen psychology and biology, but I'm not sure whether I should do sociology or chemistry (or something else) alongside those?
With sociology, I did not take it for GCSE but apparently half of the sociology a-level students last year hadn't either. Some people have said I should take it to compliment psychology, others have said it's too similar to psychology and I should pick one. I've heard it's quite content heavy and the others are too so I don't know if that might be a problem?
With chemistry, I'm okay at it and I'm predicted an A at GCSE but I'm worried the a-level would be too hard for me, and I don't enjoy it as much as other subjects. Good universities seem to like it though...

Has anyone got any advice for me?