Coach Aurelio Vidmar said the decision to let the three leave the club, which is joint leader of the A-League ladder along with Melbourne Victory ahead of the final round to be played this weekend, was extremely difficult.
"It's a very difficult decision for the coaching staff because all three have contributed positively during their time here but unfortunately with the direction the club is moving into, they are not part of our plans," he said. "However, all three will still play a very important part in our finals campaign as we try to win the championship."
Brazilian playmaker Diego has made 28 appearances for Adelaide since being signed from Miami FC in January 2007 while 26-year-old Salley, who is from the Ivory Coast, has spent two seasons in Adelaide.
Former Socceroo Costanzo, who has played 73 A-League matches for Adelaide, will be departing along with Paul Agostino and Michael Valkanis, who are both retiring, while defender Sasa Ognenovski will head to South Korea to join Seongnam Ilhwa.