Bulbapedia explains it all better than I could.
[In] Pokémon Black and White:
The player can encounter Cobalion in Mistralton Cave's Guidance Chamber, which requires the use of Surf. Encountering Cobalion will enable the player to encounter Terrakion in Victory Road's Trial Chamber and Virizion in Pinwheel Forest's Rumination Field. When gathered at the Moor of Icirrus, Cobalion, Terrakion, and Virizion will teach Keldeo its signature move, Secret Sword.
[in] Pokémon Black 2 and White 2:
The player can now encounter Cobalion at Route 13, Terrakion at Route 22 and Virizion at Route 11. When gathered, Cobalion, Terrakion, and Virizion will teach Keldeo its signature move, Secret Sword at Pledge Grove instead.
[in] Pokémon Omega Ruby and Alpha Sapphire:
The player can encounter Cobalion, Terrakion, and Virizion at the Pathless Plain while soaring in the sky as long as there are three Pokémon with 510 EVs in the player's party. Cobalion appears on Wednesday, Friday and Sunday; Terrakion appears on Tuesday and Saturday; and Virizion appears on Monday and Thursday.
[in] Pokémon Ultra Sun and Ultra Moon:
The player can encounter Cobalion, Terrakion, and Virizion in the Ultra Space Wilds' Rocky World dimension by traveling through green warp holes in Ultra Warp Ride.
[in] Pokémon Sword and Shield:
The player can encounter Cobalion, Terrakion, and Virizion in the Crown Tundra after collecting 50 footprints of the respective Pokémon. Sonia will then pinpoint the location of the Pokémon, making Cobalion, Terrakion and Virizion appear in Frigid Sea, Lakeside Cave and Giant's Bed, respectively.
After capturing Cobalion, Terrakion, and Virizion, the player can also encounter Keldeo. After interacting with a pair of footprints near a pot on an island in Ballimere Lake and making a curry with Cobalion, Terrakion and Virizion in the party, Keldeo will appear on the island.
[in] Pokémon Scarlet and Violet:
The members of Swords of Justice are obtainable after completing the main story of The Indigo Disk. They can be encountered in Paldea after obtaining Cobalion, Terrakion, and Virizion Treats from Snacksworth. These treats can be obtained in Violet through solo Blueberry Quests, or in Scarlet by participating in group quests through the Union Circle. Additionally, the games introduce supposed descendants of Virizion, Terrakion, and Cobalion: the Paradox Pokémon Iron Leaves, Iron Boulder, and Iron Crown respectively.
[in] Pokémon Legends: Z-A:
In the Mega Dimension DLC, the Swords of Justice can be obtained in Hyperspace Lumiose within unknown 5★ distortions that appear by performing Special scans.
[in] Pokémon Black and White:
TornadusB/ThundurusW will start roaming around Unova after the player listens to the story of an old woman on Route 7. If the player possesses both in their party, they can encounter Landorus at the Abundant Shrine on Route 14.
[in] Pokémon Omega Ruby and Alpha Sapphire:
The player can encounter the forces of nature while soaring in the sky. TornadusOR/ThundurusAS can be encountered in the Storm Clouds located near Fortree City if the player has a Castform in their party. Landorus can also be encountered in the Storm Clouds located near Fortree City as long as both Tornadus and Thundurus are in the player's party.
[in] Pokémon Ultra Sun and Ultra Moon:
The player can encounter the forces of nature in the Ultra Space Wilds' Cliff World dimension by traveling through red warp holes in the Ultra Warp Ride. Tornadus is exclusive to Ultra Sun, Thundurus is exclusive to Ultra Moon, and Landorus appears only if both Tornadus and Thundurus are in the player's party.
[in] Pokémon Sword and Shield:
In The Crown Tundra DLC, the forces of nature are among the Legendary Pokémon that can be encountered in Dynamax Adventures in the Max Lair. Tornadus is exclusive to Sword, while Thundurus is exclusive to Shield; however, participating in a Dynamax Adventure hosted by a player of the opposite version allows both to be obtained. Landorus appears in both versions.
[in] Pokémon Legends: Arceus:
After the player receives the request "Incarnate Forces of Hisui" from Cogita, Tornadus, Thundurus, and Landorus can be found roaming Hisui in certain weather conditions. Tornadus can be found in Bonechill Wastes during a snowstorm, Thundurus can be found between Sand's Reach and Lunker's Lair during a thunderstorm, and Landorus can be found on Ramanas Island during sunny weather. After the player completes the Pokédex entries for all three and reports back to Cogita, she reveals that there is a fourth member of the group, Enamorus, which she presents before it leaves. Enamorus can then be found in the Scarlet Bog (in any weather). After Enamorus's Pokédex entry is completed, Cogita rewards the player with the Reveal Glass.
[in] Pokémon Scarlet and Violet:
All four members of the forces of nature can be transferred into the games through Pokémon HOME.
