There are total of 56 collectibles in Black Ops 3, spread throughout the entire campaign. The Intel collectibles are gone, we have now 56 unique collectibles (see list below). All collectibles are saved automatically when picking them up. If you miss one of these collectibles, just load the respective chapter.
In your Safehouse (Campaign - go to Safehouse - Collectibles) you can see each unique collectible which you have collected and which not (empty place with mission name)
Here is the complete overview from this section (left to right) so you can find your missing collectible easier with the timelines below.
Small Collectibles:1st Row: 54i Data-Pad, Bio-Luminescent Orchid, Bomb Detonator Cap
2nd Row: Broken Respirator, Child's Toy, Coalescene Marketing Material
3th Row: Defective Robot Hand, Dragon Necklace, Dud RPG
4th Row: Dud Slave Collar, Egyptian Army Cap, Etched Glass Bottle
Medium Collectibles:1st Row: Antique Vase, Bio-Containment Canister, Bullet Shell Necklace
2nd Row: Contamination Test Kit, Decorative Lantern, Fulgurite
3th Row: HCXD Bomb Sniffer Robot, Hamsa, Hookah
4th Row: Industrial Drillbit Parts, Piece of Broken Statue, Prototype DNI Implant
Large Collectibles:1st Row: Broken Stained Glass, Damaged Optical Camo Cloak, E-Ink White-Board
2nd Row: Military Officers Sword, NRC Propaganda Poster, Promo Poster
3th Row: VTOL Panel, Wall Hung Carpet
In the video you see the counter for the collectibles, the mission objectives, the name of the mission and the name of the specific item which is currently collected in the left upper corner. In the last spoiler is the video again so you can use the other spoiler without scrolling the entire side.
Mission 1 - Black Ops
Mission 2 - New World
Mission 3 - In Darkness
Mission 4 - Provocation
Mission 5 - Hypocenter
Mission 6 - Vengeance
Mission 7 - Rise & Fall
Mission 8 - Demon Within
Mission 9 - Sand Castle
Mission 10 - Lotus Towers
Mission 11 - Life
Video again: