Pin 3 should connect to two wires. One goes as ground to the input socket and is often the screen of a screened cable. The other wire goes to zero volts on the PSU to “tell” the PCB where half way between the two supply rails is.
In a stereo amplifier this will result in a ground loop unless specific wire routing is used. The stereo wiring diagram is shown below. Note how the two zero volt ground reference wires go side by side to the input sockets and then lie closely alongside the twisted pair(or screened cable) on the way to each PCB. The two phono sockets grounds are ideally connected together at the input socket (not shown on diagram).