Let's be honest: we don't always understand what's happening on Game of Thrones. We've got the tangled family trees and the massive battles under control, but some of the rules regarding the show's magical, mystical elements are still kind of murky. One of the most important mystical storylines has been Bran Stark's gradual transformation from an ordinary boy into the omniscient, slightly creepy Three-Eyed Raven. We've got some major questions about how this whole process works, and whether Bran is the only person capable of the transformation or not.
In essence, the Three-Eyed Raven is a single entity, not a common title for a group (like the Faceless Men, for instance). His real name is unknown, but he was once a greenseer, one of the wise men who had the ability to see the past and the future alike. In the early seasons, he appeared to Bran as a literal three-eyed raven, and later, he was a mentor, both remotely and in real life, to Bran's development of his own greenseeing powers. Eventually, the Three-Eyed Raven, who has been appearing in Bran's visions as an old man, is killed by the Night King, who has also "marked" Bran.
After the old man's death, it seems that Bran is more closely associated with the Three-Eyed Raven persona. His uncle, Benjen Stark, even says outright, "He [the Three-Eyed Raven] lives again." This seems to suggest that the persona of the all-knowing, powerful Three-Eyed Raven is something that's passed down, seemingly from one seer to another, with only one person holding the title and full power at a time. Theoretically, then, anyone could take on the mantle, but it's unclear exactly how the process works or how the selection happens. After all, we met at least one other greenseer in the series: Jojen Reed, who first introduced Bran to the concept way back in season three. We'll probably never get a full explanation of how the Three-Eyed Raven situation works, but we do know that it's pivotal to the coming battle against the Night King!