Typically over heating, running rough, and white smoke combined all relate to coolant consumption. If the coolant level is consistently going down with no traces of a leak and you have thick white smoke from the tail pipe then its pretty clear there is a coolant consumption problem. Pulling the spark plugs and looking for which plugs are essentially steamed clean and look perfectly clean will point you in the direction of which cylinders are consuming coolant. And from here the only way to 100% know the root cause of the problem is to tear down the engine. Repairs like this typically go smoothly if the severity of the vehicles symptoms are correctly conveyed to the customer right from the start, repair vs replace is critical! good luck!!