I have a wall at the foot of my stairs that has 3 vents in about 5 ft of wall. One of them ducts straight to the HVAC system, the other two just open into the unfinished storage/utility (furnace, water heater) room on the other side. I'm curious as to why they built it with so many vents.
The high one is your return to HVAC. The small one down low is the supply from HVAC. (This arrangement could be reversed depending on your climate.)
The low large one is probably a "makeup air" vent, which allows combustion in the utility room to draw fresh air for its needs. It could also just be ventilation to prevent staleness.