This happened on my '93, turned out it was the brake lamp in the middle that burnt out, the one on the back window. But you said you checked all three so I'm assuming you checked that one too so I don't know that it would be a light bulb. Check your brake fluid, if it's too low, the light will come on. If that doesn't do it I would go through and check all the lamps again and make sure your e-brake is down all the way.