I'm not sure you will achieve 2-3km range with PLC devices, which are typically designed for use within a house.
Also, these PLC modules cause a large amount of interference to many devices which use radio, as power lines were never designed to carry high data rates and so much of the energy is radiated - they act like a giant antenna.
If you only need to send a 1-bit signal (on/off), perhaps you could use a relatively cheap pair of walkie-talkie type radios (maybe with a directional antenna or mounted as high as possible). Wire your input signal to the transmit switch for "on", and on the receiving end, it shouldn't be hard to find a signal which goes high when the radio receives (probably the power to the audio amplifier will be switched on). That would give you your on/off signal.