Question: how to get the key on level b2 in luigi's mansion 3?
Answer: This section of the Luigi's Mansion 3 Guide reveals how these events unfold. When you arrive in the Boilerworks, go to the right and, after the cutscene, suck up the tarp on the back wall (pictures1-2). Once this is done, send Gooigi into the pipe and get the key, then open the door on the right (picture3).