What are typical systems if the brake lights come on & then go off within 3 seconds? Could it be that the brake fluid is low or something else?
It most commonly is the brake fluid level sensor letting you know its time to inspect your brake system. You do not nor should you need to add any fluid.
Why shouldn’t I add more brake fluid?
The brakes operate with a sealed hydraulic system. You do not need to add fluid. When the pistons of the caliper move and the brake pads wear down more fluid is needed to maintain there use. Once they wear down so far the light comes on. When you replace the brake pads the calipers pistons are moved back in requiring less fluid. When the pistons are pressed in to accommodate the new pads, the brake fluid level rises back to level.