Top Lever.  The top lever controls the amount of fresh air entering the car. With the lever all the way on the left, no air enters. As the lever is moved from left to about half-way, the vents open and let fresh air in.  As the lever reaches the sections marked I II and III, the fresh air blower (mounted in the front trunk under the windshield)  blows air at increasing speed.  With the lever on the right, the blower motor is blowing air at full speed and all the vents are open.  Additionally, the fan light on the right side of the top lever lights up when the fresh air blower is on

Middle Lever. The middle lever controls the location of the flow of air. With the lever all the way to the left, the air is directed to the passenger's feet.  As the lever is moved to the right, the flow of air becomes split between the upper vents and the floor.  When the lever is all the way to the right, the air is blowing only through the upper vents.  On the later cars, (72-76) the air is blown through both the vents on the windshield, and also the dash vents.

Lower Lever.  The lower lever directs the flow of heat.   This lever is only meaningful if the heat is turned on with the lever on the floor behind the gear shift. The lever works the same as the middle lever, but for the flow of heated air.  Turning the lever to the left blows hot air in the footwells.   Turning the lever to the right blows hot air directly at the windshield and through the vents in the dash on later cars.

     Any variation on this seen in your car is a clear indication that one or more of your vents are not working and either need to be fixed or replaced.