Apparently there are small rectangular exit holes on the bottom of the door. I got 2 washers out and the third one probably fell through and rolled away somewhere.
When installing the triangle piece of the mirror, put the screws through it first and then the washers on the back. Do not remove the screws or the washers will fall through the door. Japanese ingenuity for the success!
Those things are actually steel and can be picked up by a magnet but because they are so tiny and weird, the magnet stick has a tough time grabbing them.
So, anyone got a spare?