Did the plumber tell you depth to water, and show you why he believes that the pump should be pumping 3 GPM, and how he determined that it is pumping only 1 GPM.
Did the plumber measure the pressure/vacuum on the suction side if it is a shallow well jet pump? Comparing that to the depth to water is a good way of determining if the screen is restricting the flow.
What kind of pump do you have? Deep well jet? Shallow well jet? Horsepower?
A 3 GPM pump is a pretty small pump. What make/model is it?
All in all, is the plumber speaking from facts and knowledge of the system, and is he explaining those facts to you, or is he pulling something from his hat or some lower location?