Is my low compression from bad rings?

My 240D has a compression problem. #4 cylinder is good (320psi) and compression on cyl's 1-3 are a dismal, but identical, 200psi on the nose. before I rip the engine apart to R&R the rings, does anyone know of another explanation for this symptom? I just did a valve adjustment and i have verified the compression results through both the glow plug holes and the injector holes. Does anyone know of something else i can check before the rebuild to make sure it is necessary?

any help would be greatly appreciated!