This is from my perspective only, I am someone who takes Physics, Chemistry and Biology at A2 level, same for AS (didn't do an extra one).
I can tell you it is a LOT of work. If the teaching at your college is as bad as it is at mine (and that the students are as lazy), expect a lot of people around you to fail, and expect to have to self study your way through it (which I actually prefer doing). But obviously if they're good, that's a null point.
Don't let other people tell you what you can and can't handle with respect to workload, because everybody is different. By no means is the workload "unmanageable", not in the slightest. And boy does it feel good once the hard work pays off and you come out with good grades.
Also don't let people tell you you're working too hard, etc, or follow the group's general example - you need to use your own best judgement. I had plenty of people do that to me. A lot of them failed.
They do compliment eachother very well, moreso at A2 (eg nitrogen cycle and polypeptides in Biology coming up in Chemistry), and on occasion some parts of the syllabus are basically the same (eg the e=hf photon energy stuff in Physics also comes up in chemistry), which is always a nice surprise.
Don't be deterred by the amount of rediculously intelligent people on this site who are taking like 5 A levels and getting A*s across the board. Because there is a very high concentration of those people on this board compared to in real life. Don't let them tell you it will be "easy" either, because it won't be, unless you are incredibly capable (and I see plenty of comments to this effect every day).
Also don't be deterred by people saying Biology is the hardest, Chemistry is the hardest, etc. Everyone finds them differently (as evidenced by a poll on here not long ago where it was pretty much a random assortment). I personally find Biology by far the easiest (and most fun), and Physics to be by far the hardest. I was very surprised to find plenty of people have it the other way round.
Oh yeah, as regards to a fourth subject you find "easy"/"relaxing" - in my opinion, don't. It will get extremely irritating when you have work due in for that "fun" subject when you have work to be doing on your main subjects and could potentially end up losing marks because of it. Imo, if you're gonna take a fourth subject, choose one that's related in some way, or just stick to focusing on your three priority subjects, like I have done.
Hope that helps!