Im not sure if GCSE psychology helps with A level psychology but its true that Psychology A level theres a tonne of stuff you have to remember. If your doing AQA like I did, there were like over four hundred evaluation points that pop up randomly... But to be honest it may look scary from the get go, but when you start learning and making connections, it becomes easier to remember.
However, with chem and Bio you have to remember a lot of facts also, so pairing it up with an essay A level is a bit out there, perhaps a little stressful. I was comfortable with essays since i was doing english which is way longer and there is 1 essay in Biology AQA also, so Psychology was really nice. But if you dont feel like you can write 6 detailed snappy paragraphs in 15 minutes, probably 3 times, whilst remembering all the details and at least 4 evaluation points ( In this years exam, one paper had 3 essays), then psychology might slow you down. A lot of people don't get to finish. Its about time management.
But, I;ve said this before, Biology and Psychology complement each other well. They overlap a lot as well so it gives you an edge. Theres a whole section of AQA psychology which focuses on Biology called Bio-psychology.
I think picking Maths as a 3rd is what I would do in your situation. Typically, people who want to do medicine do chem bio and maths, its like the generic trio lol. Plus Maths is not racing against time with long ass essays. If you have a knack for maths, it may ease the pressure of the 2 other subjects which are to be honest very challenging. But if you have a creative side and are very good at memorising as well as writing fast and in a sophisticated way, then Psychology is for you.
Even if it may take more brain and memorising, Psychology will only ever ask questions that are in the book and you know you memorised. Its basically just vomiting out information in your head, unlike Biology where they ask you stuff in a roundabout way. So if you fully memorised everything, the paper can only ever be easy. It will never be hard.