Hiya I do this exact combo bio, psych and maths
Im a few months into year 12 so don't think I'll be that much help but all I can say is biology is VERY content heavy and so is psychology. I'm currently struggling with maths and I got an A* in GCSE maths (did it a few years early) but I think that's just a lack of practice on my behalf. Personally i find chemistry wayyy harder than maths and it is one of the hardest A levels but so is maths.
What careers are you looking into? Sorry I couldn't be much help please do ask any questions and I'll try my best to help.