Johnson is the only guy on the whole roster who can defend someone like Brook Lopez, who was the main reason the Cs lost to a team with a 25% win pct.
Turner is the one of two players on the roster who can handle the ball, get assists, and get easy baskets at the rim.
Lee is a veteran and former All-Star you know is talented, and seemingly a good team player.
I went to basketball reference to check out our best net 5-man combinations thus far this year. Take a look for yourself:
http://www.basketball-reference.com/teams/BOS/2016/lineups/Our best 5-man combinations are littered with Bradley, Crowder, Thomas, and Turner. So there's your reason for Turner. Lee just happens to be in the top 2, so not sure how that's evidence against him. Johnson can be found in a couple as well, but he is clearly the weakest link of the 3 you cite here.