We have 1 shower that cannot maintain hot water. I can get hot water for about 2-3 minutes and then it goes to freezing cold.

My husband uses a different shower in the morning and there are no issues. There are also no issues with any faucets.

We have a tankless water heater.

Is your house on slab ? Where is the heater in relations to your, and your husbands', shower ?

No slab. We have a full basement, but the heater is on one end of the house, while the showers are approximately centered in the house.