My advice is leave existing alone and add another return strategically located and sized. Rule of thumb is you need 400 CFMs for each ton of cooling. Adequate return improves airflow and comfort.

Yes! Add another 20x25 return, attach it to system with 18" duct so as not to impede the air flow.

Best method is to locate this in an area with warm air away from original return. A known hot spot within the structure. Often times this will be near an entry door into the structure.

Thanks. Filter is also 20x25. No measurement of static pressure that I'm aware of. Did measure air flow at something like 920cfm, so considerably less than 400 cfm/ton. Sounds like if it cools well (and won't know that for sure until summer) to leave well enough alone.

In general you can support up to 2CFM/sq in of filter. If you have 920 CFM, which is low for 3 tons, then 920/2 = 460. Your filter is 500 sq inches so you should be fine.

920-CFM for 3-Ton is only 306-CFM per ton of cooling, that is too low; minimum with a piston evaporator metering device ought to be 350-C F M per ton; 400-CFM per ton is bet if no humidity problems. Not adequate for 3-Ton needs duct work performed, more return ducting & perhaps also more supply... If the air handler was/is 2.5-Ton it my not have blower capacity enough for -3-Ton...?