I have several rental properties where we have installed varying brands of tankless water heaters, gas and electric. We have great hot water at kitchen and bath vanity faucets, but the tub faucets run cold. I assume I need to constrict water flow so the system will be activated. How can this be done on a tub faucet to change waterflowfrom 3gpm to 2 or even 2.5? Water-saver shower heads resolve the issue in the riser.

The tankless heaters detect water flow and turn on when flow is detected. The opposite from what you stated was a common issue. Sink faucets that didn't flow enough gallons per minute(water conserving aerators and not having the faucet fully open) wouldn't activate the heater. The tub should flow more than a faucet and activate the heater. There's something else going on. If there's a hot limiter in the tub valve, maybe the setting is too cold and not enough "hot" water is being called for, for the heater to turn on?

yeah, the heater will slow down the flow so you don't run out of hot water. (when functioning properly)...