The average opening in a unit return suction air side is 21″x21″=441sq.”
You have a 5-ton horizontal gas furnace and you see (1) 2″, (2) @ 10″, and (1) at 6″ return: Is it enough? NO
A return duct system minimum is to be the opening of the end! To be 25% to 100% larger will help in filtering air particulates.
(1) 12″+ (2) @10″+ (1) @6″= 441.0, sq. in.
113″+78.75″+78.75+28.26″= 298-Msq. in.
Your duct work is 142.24 sq. in. short of opening size.
This condition reduces the effectiveness of your unit and causes air restriction. Plus, as your filter clogs up, your air conditioning or heating system will need to run longer to do its job. This is a direct energy cost. Also, by not having enough return duct work for the size of the system, you can cause premature compressor failure! Now you are looking at a substantial expense to repair.
Important Note: Make sure you have enough return air or hire an HVAC contractor to add duct work to provide more.