They have been moved around the globe on a monthly and will now soon be completely taken out of the Gym Raid cycle.
November 30 is the last day scheduled to include all three Pokemon to be available to catch, after which they will likely be removed from the game.
Currently, Suicune is available in the Americas; Raikou can be caught in Europe and Africa; and players in the Asia-Pacific region have their final chance at adding Entei to their collection.
Niantic have not yet said if they have plans to add new Legendary Pokemon to the existing EX Raid system, or whether that will remain exclusively for catching MewTwo.
But while Entei, Raikou, and Suicune will soon be removed from the Gym Raid playlist, Trainers will still have access to one Legendary-type.
Pokémon originally discovered in the Hoenn region made their first-ever appearance in Pokémon Go during the Halloween event.
During that event, Niantic confirmed that the rest of the Pokémon first seen in the Pokémon Ruby and Pokémon Sapphire would start arriving in December.
However, this is set to be a gradual rollout, meaning that we might only see a few arrive next month.
This could be part of a new seasonal event being launched by Niantic, although there has been no confirmation of their plans yet.
December 1 will also see the end of Pokemon Go’s Travel event.
After successfully catching more than 3 billion Pokemon, fans earned double XP and double Stardust in Pokemon Go.
Presumably this also means that special item boxes will remain on sale in the store.
To make battles a little easier, Pokemon Go fans can currently pick up Super Incubators for 200 PokeCoins, Special Boxes for 380 PokeCoins and Ultra Boxes for 880 PokeCoins.
Costing only 50 PokeCoins more than regular Incubators, Super Incubators can be used to speed up egg hatch rates.
Special Boxes, meanwhile, contain 20 Great Balls, six Lucky Eggs, four Lure Modules and one Premium Raid Pass.
Finally, Ultra Boxes come with 16 Lucky Eggs, four Premium Raid Passes, eight Lure Modules and a whopping 30 Ultra Balls.