3 hours is done in a single session. I do it like that for reasons of convenience and also that to some extent a marathon race is just like every other day. Since I just run laps providing water would be easy but in fact I do not drink on my training runs even in the hottest weather. Typically I do not drink in races, if it is hot I will drink a bit in a half marathon. I generally drink a bit in a marathon, more if its hot.
I'm still curious: assuming 3 hours a day, do you do it all at once or break it up, and if all at once, how do you manage water intake?
Also, when you do race, what is your perceived level of effort relative to your training jogs?
I don't know where you are right now, but I hope your course didn't get flooded out!