Turn off the engine immediately if it’s safe to do so. Then call breakdown cover, RAC Fuel Patrol or the like to drain and flush the fuel system. Putting petrol in a diesel car can cause serious damage to the fuel injection system and engine. A diesel fuel pump operates on very fine tolerance at high pressures and is lubricated by the diesel fuel. When petrol is added to diesel it reduces its lubrication properties, which can damage the fuel pump through metal-to-metal contact and create debris that causes significant damage to the rest of the fuel system. It can be a very costly mistake so you'll need an expert to drain the system. I can't predict how much or how little damage you've done by driving it 3 miles, though.