i reckon all subjects are equally demanding in different ways.
i didnt do ancient nor modern but my friends said that you have to do lots of research and essays. nonetheless they enjoy these subjects. some also say that modern is a way better than ancient as well. so, if you're quite good in writing essays etc and enjoy this sort of thing, do history.
on the other hand, with physics, you are less likely to write essays... but you have have to do a bit of research as well (more like a report assignment on a particular scientist, etc), prac work, lots of homework (you have to be consistent coz if you miss somethiing then you're pretty much screwed). but then if you're more of a scientific person, then do phys.
if you're stil confused, check the syllabus for next year (im not sure if they're available just yet) or talk to the teachers and ask what sort of topics you'll be doing for each of these subjects and see which ones interest you more...
OR
you can pick all 3 LOL LOL LOL