You should only run a single openHAB instance on the master RPi3.
First, you should rethink if the Pi0 is the right actuator for whatever you want control with it. There’s many other devices out there like Sonoff or whole subsystems like ZWave and ZigBee.
If you still want to use the Pi0 then run a script there, talking MQTT to the master Pi.
Check out the links in this post and search the forum for MQTT howtos.
