The larger cylinder is an electric coolant pump. If a coolant temp in excess of 110 C is sensed after engine shutdown, this pump will start and run for about 10 minutes. Coolant is circulated through the engine to prevent hot spots.

The other two cylinders are pulsed valves. They receive inputs via the cabin temp selector and regulate coolant flow through the heater core.

The MB service information describes a procedure to insure this part of the coolant circuit is open for flow during a coolant flush procedure. Sorry I can't remember the exact details.