My water heater is about 20 years old and I can only get about 3 inches of hot water in my bathtub before it runs out. What is the most probable cause?

Do you start with JUST the hot water? When was the last time you checked your dip tubes, flushed your tank of sediments, or any other normal routine maintenance? How big is your tank. How long has this been happening? Gas or electric? What is your tank temperature set at?