Are you taking in carbohydrates during your 3 hour ride, you should take in enough carbs before and eat throughout the ride. Getting a good base knowledge of drinking before and during rides would be a great start, also I would find a sports drink that has the right % of carbs for the time you are riding,it would be a good start. Followed by some reasearch on eating/nutrition during your ride.
As mentioned, nutrition and fluid intake should be started 1-3 days before, to increase stores and to ensure your tank is full.
Everyones needs will vary but there is some good info out there on how to work out your requirements, Just make sure you work out your own requirements, dont use someone elses.
Maybe your training could be looked at as well, there may be some tweeks or changes that will focus and improve your ability to ride for 3 hours.
I have a copy of the time crunched cyclist's which I have found quite good, It focuses on training with only 6 hours a week, although I dont use it for training it has been very helpful for information.