Top = Control to outlets on the top of the dash.
Middle = Cold when at the left to Hot at the right.
Lower = Control to outlets by drivers left knee and passengers right knee.
If you're talking about Aircooled then your van probably was, or still is, a Bus and the 4th controll is to open the flaps which direct the air to the vents in the back for the passengers.
So one lever controlls diretion , feet or windscreen. One turns the heat on and off by opening the heat exchanger pods. One opens the fresh air vent and one directs air to the back.
Again if it's Aircooled then you don't actually need the fan but some Buses were fitted with them to give the airflow a bit of extra oomph.
If it's watercooled then disregard above and I have no idea.