I would do a service on it. I wouldn't flush it at this point, I would pan drop it. Change filter and the 7-8 quarts of fluid. If you were looking to help that 3rd gear along, I'd add a bottle of lubeguard or Lucas stop slip.

At 161K miles I did the first service on my Regal trans. Literally it's first service based on the overall neglected maintenance. (air filter had 65K written on it). I'm currently at 192K and while I'm having an occasional issue..at 192...it owes no one a thing.