When equipping a person with a ring or necklace, what is the third slot for?

The third slot is for your artifacts. Played the game before while an event was going on. Then I realized I got something called "Artifact". Equip it onto your character and it will make the dweller powerful while giving your allies good effects and your enemies negative effects. Make sure you check your level of your artifact to. If it is around 60-75, and yor people are below or above the requirements, it wont allow you to use it.