The headlamp assembly that has 3 bulbs is the newer style. Maybe your car has had a replacement due to a broken headlamp lens, or a fender-bender? I don't know if the old 2 bulb style lamps are even available from MB anymore.

You cannot change the bulbs out - the reflectors are different styles and the housings won't accomodate the larger 2-filament bulb - plus I'd bet the light pattern would be off by a long-shot.

Just to confirm my suspicion - when you turn on the fog lights, where does the light come from. I'll bet on the headlamp with two bulbs, it comes from the reflector closest to the grill. From the other lamp, it probably appears below the main reflector - am I right?

I'd see about getting a new lamp assembly off e-bay. You'll need to get with a seller that's willing to confirm the lamp configuration. I'm pretty sure that the older style has a single large access door on the back, and the newer style has 2 doors. You can also try Nationwide Auto Recyling. They have several 92-93 era 140 chassis cars they're currently parting out. I've bought a bunch of interior parts for my car from them. Their prices are reasonable. If you don't see what you need, call them.

Good luck. PM me if you have further questions.