If it’s a stormcore 60d, it does nothing (they messed up wiring the original batch)
If it’s a 60d+, it does the same thing as the other uart port. It just allows you to connect another uart device. For example, you can get remote telemetry from one port and plug a data logger like robogotchi or metr into the other.
Quick note, you have to be on FW 5.3 or higher to activate the uart3 port on the 60d+