Tablets are far less harsh on new plaster than liquid chlorine would be since there really isn't an accurate way to measure very low levels of liquid chlorine (<1) additions. What they are telling you is simply not correct.
Tablets release chlorine and CYA very slowly. Any chlorine added is also somewhat buffered by the CYA that is also slowly being added. Had liquid chlorine been added to 2-3 ppm, that would be far more harsh since there isn't an adequate amount of CYA to buffer the strength. With that said, 3ppm of chlorine will not cause rust stains to magically appear, disappear and reappear elsewhere. You have something else going on.
What happens when you brush the spots?
Crush a vitamin C tablet and rub it on a stain, what happens?
What are the current test results? Are you responsible for maintaining the pool?
Did you have any rust spots on the gunite prior to plaster?