(3) Glass is transparent, and light can spread in transparent medium, so you can clearly see the outdoor scenery in a room with soundproof glass windows. The propagation speed of light in vacuum is 3× 108m/s, while the propagation speed of sound in air is 340m/s, which is 0.
So the answer is:
(1) double glass;
(2) Vacuum cannot transmit sound;
(3) The outdoor scenery can be seen clearly. The speed of light in vacuum is 3× 108m/s, while the speed of sound in air is 340m/s, which is zero in vacuum. Therefore, people can clearly see the outdoor scenery, but can't hear the outdoor sound.