Three Isle Path (Japanese: ３のしま よこあなIsland 3 Tunnel) is a small underground passage located in the Sevii Islands archipelago. Initially a cave famous as a source of gold, this location is gradually expanded into a tunnel over the course of Pokémon FireRed and LeafGreen. Once completed, the tunnel leads to an isolated patch of grass where wild Dunsparce can be caught. The cave itself houses no wild Pokémon.
The path before its completion
Three Isle Path is located on the eastern side of the Three Isle Port. A bare cave at first, it is later revealed to be a passageway leading to a secret section of the port. The cave itself is known for its gold supply, which has attracted one particular Three Island resident, mining for gold.
By the time the player has defeated the Elite Four and obtained the National Pokédex, the gold miner will have broken through to the other side of the cave. Evidently, he has been successful in his search, as he gives the player a Nugget. Following the tunnel through, the player will discover a small patch of tall grass on a rocky cliff. This grass is inhabited only by Dunsparce.
The man mining for gold inside Three Isle Path talks about wanting to buy a house from Kanto, heavily suggesting that he is the same man who can be found living at Route 2 in Generation II and Pokémon HeartGold and SoulSilver, also giving the player a Nugget when first met. This is also supported by their identical dialogue after giving the player a Nugget.
The same man also appears in the Malie City Ferry Terminal in Pokémon Ultra Sun and Ultra Moon, once again handing the player a Nugget, as well as mentioning how he used to dig up a lot of them on the Sevii Islands.