Hi OP, I have a DS in y9 at St Olave's. And was as shocked as you were to hear about the 3 hours a night of homework. Is this from a current student?
DS seems to have a very light amount of homework most of the time, for example, no homework Monday, about 30 minutes yesterday. Sometimes there can be 1-2 hours, but that is the rare exception, not the rule for us. Like his classmates who went to various different schools, they do seem to get a lot at the beginning of y7 which can be massively overwhelming: my DD1 has just gone through that at Newstead wood this September as a new starter, and it seems to be calming down a bit now. I'd say, talking to friends with kids at private/ academy/ comps and grammars, they seem to have very similar amounts of homework - perhaps private being the most.
But as I think others have said, it can depend on the child as to how long they spend - not just if they find some aspects challenging, but how focussed they are. I remember a lot of staring out of my window when I was supposed to be doing my homework.
When DS started at St Olave's, there was information on not spending more than a certain amount of time on homework, and if it was taking too long, to have a word with the form tutor/ class teacher etc. There was a lot of support.
DS (and DD1) are quite good at managing their time (I am not sure where they got this from, not me or their father!), and often say 'oh, there would have been HW but I finished it off in class'. Some of DS's St O classmates do HW at lunchtime as they have sporting/ music commitments after school.
DS and DD have got into the habit, sometimes a little bit OCD about it, of doing the homework the day it is set, which is great, but a little stressful if they get set a lot on a certain day (due in on various different days), and they are learning to be a little flexible with that. Some boys in DS's class don't do any homework in the week, and do it all at the weekend. Horses for courses. Other boys have parents who set them revision timetables for the weekend, even weeks away from assessments: that was an eye-opener! It increasingly seems that a lot of the pressure in my DS's class is from the parents and their expectations, not school. Despite the negative press and negative perception on many of these forums.
St Olave's does seem to attract a lot of negativity, and while there is a lot of stuff in the press at the moment, for us, it's not the terrible place it is often made out to be. DS had a middling experience at his local primary, a fair bit of bullying for being a 'nerd' but has absolutely flourished at St Olave's. He has had some lovely teachers and some not so great ones - like most children's experience at most schools. When he has had a couple of blips, I have been in touch with school, and there has been very good pastoral care and support and exceedingly quick response, and good communication.
I hope this helps (and feel free to private message me with any other questions).