Three suddenly freaks out about the vault and the contents inside. He tells Five to give him the code to open the door, after which Five mentions Sarah, Three's ladylove who was found frozen in the vault and later died in Season 1. A heartbroken and surprisingly sentimental Three opens one of his lockboxes and finds a green necklace, a piece of jewelry he was going to give back to Sarah "when she got better." Awww!
(Remember, this is the same guy who cried while reading Charlotte's Web, so it's really not that surprising.)