History or Psychology A level? Watch
At the end of the day it comes down to you - which you enjoyed more at AS (or before). I studied both of them at A-level and I'll give you some pros and cons for your situation
-Psychology would probably compliment Biology and Chemistry better, being a more scientific subject. There's also a fair bit of biology behind Psychology.
-History is entirely essay based, unlike Psychology where quite a lot of marks come from small 2-4 mark answers. You may find it difficult doing Biology and Chemistry and then having to suddenly switch to lots of reading and 45 mark essay writing. On the other hand, you may enjoy that fluctuation.
-Psychology is more relaxed. (I did History, Politics and Psychology) came out with A* A* B respectively. I could concentrate my efforts on my favourite subjects, while having a subject I could chill back in - but not fall too far behind, because it wasn't exactly very hard. Doing 3 'hard' subjects may be quite pressured in the final months as you can't exactly let either History, Biology or Chemistry slip - going into an exam unprepared for those three could be very detrimental. (I did Bio AS too)
-History is 'probably' a more respected A-level. But then again Biology and Chemistry are already putting you in high regard. And I presume you want to pursue the Sciences if you're certain on both Bio and Chem? Either way History looks really good as a subject, shows good writing skills, good analysis etc etc all in one.
At the end of the day it doesn't really matter too much, as long as you're not wanting to take either further. I honestly don't know anything about your situation but I'd probably pick Psychology. Compliments the sciences and also very interesting to learn about.
I know some of these thoughts are all over the place, but hopefully some of them triggered a decision within you. Good luck!