Your offer is incredibly reasonable, if they don't go for it it's because they are wasting your time. ?650 divided by 3hrs for the 3 is round about ?72 per hour for each of them - they're getting the bloody deal of the week IMO.
Plus as you've already identified, it is likely to be much more emotionally/physically demanding, you will need to be super alert for the whole 3 hours. I'm not brave enough to do it but if I was I'd want more. I'm sure that you risk assessed the situation to your satisfaction?