3000W at 230V is only 13A, it should not be enough to cause a voltage drop, and dimming lights, especially as lighting and power circuits are wired separately from the switchboard. Are you at the end of a long country road and suffering from supply deficiencies? Is your 3-phase supply 3 legs at 35A? or 12A per leg? I would be measuring voltages at the switchboard, the controller, and the lights while the controller (and element) are both on and off to find out where the voltage drop is happening. Measure in reference to both switchboard neutral, and directly across the devices themselves. To me, it seems you either have a loose connection somewhere which is showing up when the system is under more stress (drawing higher amps), or there is a wiring error that somehow puts the controller in series with the lights
divider). A clamp meter to measure current would be of benefit too. If you are inexperienced with electrical matters you are best to seek advice from someone who is experienced.
Best to sort this out, if there is wiring error or poor connection you can either kill yourself, or burn the house down.