The worst shooting Celtics in the Big 3 era by shooting percentage (minimum 20 games) are Scalabrine, Dooling, Moore, Marbury, and Pruitt. PJ Brown, Carlos Arroyo, and Marcus Landry missed the 20 game threshold.
By true shooting percentage, it is Moore, Bradley, Harangody, Pruitt, and Marbury.
Measured by PER, the worst Celtics are Bradley, Dooling, Pavlovic, Scalabrine, and Marbury.
By WS/48, O'Bryant, Moore, Marbury, Bradley, and Dooling.
By offensive rating, Dooling, O'Bryant, Pavlovic, Bradley, and Marbury.
By defensive rating, Scalabrine, Dooling, Allen, Marbury, and Finley.
The only player on all of these is Stephon Marbury and I would gladly nominate him as the worst performing player on the Celtics in the Big 3 era.
EDIT: To clarify, every Moore is E'Twuan Moore, not Mikki Moore.