If you're looking for a new weapon in Black Ops 3, the Mystery Box is a great way to get something powerful without spending much money. It's also a fun way to play the game, though it can be challenging to find the right one at the right time.
The Mystery Box appears in all Zombies maps except Nacht der Untoten, Verruckt and Dead Ops Arcade. It can spawn in various locations, but is usually placed on a wooden table over a few cement bricks and emits a colored beam into the sky to show where it's located.
In all levels, it can be opened to reveal a random weapon, which changes its color as the item progresses through rarity (green for Uncommon, blue for Rare, purple for Epic, orange for Legendary and yellow for Ultra or Wonder weapons). Once a weapon is obtained, a small teddy bear will appear on the box's lid, and a musical jingle plays to let players know that it has been triggered.
Some versions of the Mystery Box will also contain a teddy bear or lock that causes it to shake, rise up and teleport elsewhere on the map with no useful weapon dispensed. Depending on the location, these items are usually not very valuable and may even cause the player to lose all their progress in that level.
The Mystery Box is also a key part of the map Firebase Z. However, it doesn't spawn at the beginning of the map, so you might need to head further into the base to get it.