Converting "wet" contact closure to "dry" contact closure
A dry signal is just a contact closure. 

You can use a 3V relay driven by your 3V  signal. If the 3V signal does not have enough current capability to drive a 3V relay directly you will need to add a driver transistor and supply appropriate power for the relay coil. Typically this would involve a transistor, base series resistor, diode across the relay coil and the relay itself.