Sudden Water Temp changes

I have a beauty shop with 3 sinks next to each other. Recently replaced my 40 gal gas water heater with another due to a leak. Its the same type and size as the old water heater. The problem is that when you turn on one sink and get the temp adjusted and the next person turns on the adjoining sink the temp changes radically. (Like Dont flush now, Im in the shower!) This didnt happen with the old heater. Ive already paid to have the old valves replaced that lead from the water heater to the rest of the house. Also paid to have the lines flushed. Isnt there a device that regulates pressure or temperature that can be installed under the sink. Whats it called? Is this what I need? Help.