On EXP3 there is PWM_1 located, which is defined in my Rumba_pins.h as Servo pin ( meaning it is already defined as output ) The CPU-pin# is "1"
So in theory you could send a gcode "M42 P1 S255" to switch the relais. BUT this pin is directly connected to the CPU and not very powerful. I doubt it can drive a relais.
It would be easier to use a MOSFET driven fan-output and split the output voltage with a resistor divider.