I don't believe 3hr exams would be that much of a problem for a lot of people; e.g. at college my friend has 2 hour lessons and then study time afterwards, so she has to keep concentration.
Also, my last bio exam was 2hr15 and actually I could have continued writing for longer - Once you get into the swing of things its easy to write and work for that amount of time.
It'll only be a struggle and difficult for those who haven't revised properly and so haven't got enough to write about, and therefore those who will get the poorer grades.