I’m currently a sixth form student myself, and to cut it short, all a level subjects are hard. Yes some maybe easier than others but all have the requirement that you remember the content and be more independent. For essay based subjects like mine, i would say that the workload is crazy, and teachers will expect you to rely more on yourself, also the content is really heavy and so more responsibilities are required when it comes to stuff such as revision etc..
Considering your choosing History and Geography, I’m pretty sure you are aware that both include you completing an NEA (non examined assessment) and so choosing both may be hard but that shouldn’t scare you.
My tips, choose the subjects that you feel comfortable with, make sure your three a level combinations are a good enough mixture for you to get into university, and finally, yes sixth form can be quite scary but at the same time, as long as you keep up with your work and start revising from day 1, all should be fine!
SO SORRY FOR THE LONG PARAGRAPH LOL! (Hope this helps tho)