I live in a 3 story town house, water heater in basement. When I shower on the top floor the shower water temp. after adjusting keeps getting cooler, needing to be adjusted again and again. The main floor kitchen and 1/2 bath are fine good hot water, no problem. This is a recent problem. What's wrong??

You didn't say, but I'll bet it's an electric water heater. It's possible you have an element burrned out. If that is the case, you have less hot water to start with and the "trip" the water makes up to the third floor is enough to use some of that water up.

As you use hot water, cold water enters the water heater and mixes with the hot water already in the tank. Naturally that lowers the temperature of the hot water as you shower.
However, if this is something new, then it is likely that you have a deteriorated dip tube in the water heater. Normally the cold water coming in is taken to the bottom of the tank by the dip tube. When the dip tube deteriorates or breaks, the cold water coming in can move directly across to the hot water out side.