For that 3-ton system, I would have sized the duct system for 1300-CFM; 16" metal; @ 932-fpm velocity.
For a 2.5-ton; 15" rd. main if available; 1100-cfm; @ 897-fpm. Friction Loss Per 100' of duct, 0.09 IWC.
For a 2-ton; 14" main; 900-cfm; 842-fpm.
tinner73, my software for 2-ton, 800-cfm, I get a 13" duct; @ 868-fpm; FR is 0.10" | However, I would size for 900-cfm. Though, your figures are okay.
With your figures I get; 12"; 800-cfm; 1019-fpm; FR 0.15" WC per 100'.
Size the return equal or a little larger.
Airflow is everything.
The existing 3-ton with a poor duct system, may be delivering around 1½-ton. (?)
Reduce the heat-gain first; then do the manuals J, D,& S, & stop guessing. - Darrell