1998 CHEVROLET C3500-HD ABS Sensor

I was looking into ABS sensors for a 1998 CHEVROLET C3500-HD Dually with a 6.5 Deisel thats been converted into a flatbed, the ABS light is on and the guy who owns the vehicle was told that he would have to replace them. When I talked to the parts guy at the dealership I work at he could only find one for the front left wheel. Does anyone know why this would be? I would have assumed there would be at least 3 ABS sensors on a truck that size. Any info much appreciated.