Admiral Ackbar
IG-88 can open any door. Go to the panel to the left and open the door. Then, use your thermal detonators to blow up the wall of junk. Next, build the thing next to the chamber. Ackbar should be inside.

Obi-Wan (Mission 2)
4-lom can open any door. Take him to the lift where the R2D2 control panel is located. Open it and go up the lift. Pull the levers after walking on the foot marks and enter the next room. Use the control panel again and go through to the last room. Obi-Wan is on the right. Smash through the boxes and he will be standing in the small doorway.

R2D2 (Mission 1)
Enter the Bounty Hunter area. Keep going forward, then stop when you get to a wall. Enter both holes at the side of the wall. Legos will fly out. Build it into a doorway. Inside is R2D2.