Dianna Siveny and Kandi Siveny are charged with Plamann's 2007 death, as is Rosie Campbell.
Plamann and Dianna Siveny were in a relationship, and prosecutors say Siveny believed Plamann was cheating on her. The three allegedly plotted the murder. After Campbell decided not to killed Plamann, Kandi Siveny allegedly fired the shots.
Campbell, who has cooperated with police, goes on trial May 12. The Sivenys are scheduled to go on trial May 19.