Outpost 3 was an outpost constructed on an asteroid by Starfleet in 2160. This outpost was one of seven Earth Outpost Stations located in Sector Z-6 on the Federation side of the Romulan Neutral Zone.
During the Neutral Zone Incursion, the location of Outpost 3 was labeled on a map of Sector Z-6 that was displayed on the bridge viewscreen of the USS Enterprise. (TOS: "Balance of Terror", production art)
Between 2266 and 2366, this outpost had been rebuilt. The location of the new Outpost 3 was labeled on a tactical intelligence analysis star chart. This base received a warning from Starfleet Command about an alleged Romulan base on Nelvana III. (TNG: "The Defector", okudagram)