As a former owner of a '97 E320 (now a '06 E350) here goes:

First button is for the cabin air filter. When you press the button, outside air is passed through an activated carbon filter. Supposed to get rid of bad smells (like a dead skunk).

If you leave it on all the time you will have to replace the cabin filter more often. I rarely used the button so I don't even know where the filter is.

P.S. - the AC comressor would come on when the cabin air filter was in use so it will also affect your mileage.

The second "W/S" switch is for winter/summer mode for the transmission. Winter mode starts out in second gear to reduce wheel spin. Only used it on snow covered roads.

The third "knob" is not a knob. It's some sort of sensor.

Lou