Hey there.

I have an old two knob tub faucet, Price Pfister, with the diverter as a third knob. The cold water pressure is fine, but the hot water starts weak and gets weaker and weaker as it runs.

What could be the issue?

You may have debris or the washer broken and stuck in the hot port of the faucet, shut the water off remove the stem check the rubber washer for condition, while the stem is removed turn on the water for a second to flush out all the debris, I would suggest to replace the rubber washers on all 3 stems,see the images below for info. Good luck.

John

John is correct and I might suggest that you flush your water heater tank every 6 months. Do you know how to do that?