If you want to isolate one object in your 3D scene and apply an effect to it (i.e. placing a glow around the robot but not around the spaceship), apply the ID Matte effect (in the 3D Channel category).
Click a pixel in the robot and note its ID number in the Info Palette.
Then change the ID Selection parameter (in the ID Matte effect) to match the ID number from the Info Palette.
After Effect will display the robot and mask out everything else. You can then apply any effect to they layer and it will only affect the robot.
To add back the rest of the scene, place a second copy of the original scene (spaceship and robot) below the robot layer.