The players were not included on a 25-man roster for two friendly matches against Serie-A teams in Italy this month.
Thomas, a 31-year-old central defender, played 15 games this season but saw little action after the mid-season signing of Alessandro Nesta and Nelson Rivas's return from an injury.
Gardner, 30, was the starting left back to begin the season but was supplanted by Jeb Brovsky and played in only six matches.
Thomas and Gardner will be eligible for the MLS re-entry draft. The 22-year-old James, of Mississauga, Ont., did not get into a game.
The Impact had already seen goalkeeper Greg Sutton and forward Eduardo Sebrango retire. They are expected to promote players from their academy next season to fill some of the vacant spots.
Five players from their under-21 team _ goalkeeper Maxime Crepeau, defender Maxim Tissot, midfielders Wandrille Lefevre and Zakaria Messoudi and forward Mircea Ilcu _ will make the Nov. 5-17 trip that will include games Nov. 8 in Bologna and Nov. 15 in Florence.
The travelling roster includes goalkeepers Evan Bush and Troy Perkins, defenders Brovsky, Nesta, Hassoun Camara, Matteo Ferrari, Dennis Iapichino, Karl Ouimette and Zarek Valentin, midfielders Bryan Arguez, Davy Arnaud, Patrice Bernier, Calum Mallace, Justin Mapp, Lamar Neagle, Sanna Nyassi, Sinisa Ubiparipovic and Collen Warner and forwards Marco Di Vaio and Andrew Wenger.
Rivas will make the trip, but only to continue treatment on his left knee, while midfielder Felipe Martins will stay home to recover from sports hernia surgery.
The 23-year-old Arguez spent most of the season on loan to Edmonton of the NASL but remains in the Impact's plans for the future.
No decision was announced on forward Miguel Montano, who was loaned to a club in Colombia after appearing in three games for Montreal.Suggest a correction