Connect the relay to an output pin.
In Mach3, go to Ports and Pins > Output Signals, and assign that pin to Output #1.
Then go to the Spindle Setup (still in Ports and Pins), Uncheck "Disable Spindle Relays" and assign Output 1 to "Clockwise (M3)". You may want to add a few seconds of delay to "CW Delay Spin Up" So the router is up to speed before it starts cutting.
Now M3 should start your spindle, and M5 should stop it.