In you request configuration make sure that you enter the first register number as 325, which corresponds to PIN 3 status, not the register address, which s 324. Depending on how many registers you want to read at once, you may enter register count as one, to read a single 325 register, or 4, to get values from registers 325, 326, 327, 328. Also, check if you have selected Data type as 16bit UINT, high byte first and function as Read holding registers (3).
You can toggle pin states in the WebUI Services -> Input/Output -> Status section.
Below is a picture of possible request configuration: