I am just a home owner...not a HVAC pro. Sharing the setup in our home with gas heating (built in 1987) on our first floor we have two returns at floor level located kinda at opposite corners of the first floor in two different rooms (living room and family room). There is a third return that is located upstairs at ceiling height at the top of the stairs near where we can look down into the family room.
Both the family room and the living room have vaulted ceilings. The stairway to upstairs is opened on the living room side. There is a large opening with railing that looks down into the family room.
I can only guess the physics regarding the location of the three returns is possibly to recirculate the cooler lower air on the first floor with the much warmer air at the top of the second floor...to uniform the air temperature throughout the house.