According to an extensive interview with GamesPlanetAU, Black Ops 3 will allow players to customize their weapons in ways never done before in the all new GunSmith section of multiplayer.
The Gunsmith allows players to customize and build out their weapons incredibly precise then ever before. You will be able to add five attachments and optics to base of the gun.
In addition, an all new Paintshop feature will let players create camos and place them on their weapons. Paintshop allows up to 64 layers to be placed on three sides of a weapon, on top of usual paint jobs. The final game will, reportedly, feature carbon fiber material to add on. The game will still have designed camos, and players will be able to add their own paint jobs onto the preset camos, to truly personalize your weapons like never before.