While I know that the playable characters from Birthright are, for the most part, not playable in Conquest, what about Revelations? Do you simply get all the characters from both sides in Revelations or are some characters only available in one of the three versions?
Yes, nearly every character in both Birthright and Conquest are playable in Revelations, the only exceptions being Izana, who is gained by upgrading your My Castle hot springs to level 3 after a certain point in the game (you can check his wiki page for spoilers on when specifically) and certain capturable characters that are unique to the Birthright and Conquest paths, which are noted here. For a full list of characters in the game, including NPCs featured in the game, capturable boss characters, and children, you can look here. It also shows which ones are unique to Birthright and Conquest for those interested.