Which Pokémon games began Generation III?

Pokémon Ruby and Sapphire began Generation III.

Game Freak developed Ruby and Sapphire for the Game Boy Advance, releasing them in Japan in 2002 and internationally in 2003. They introduced the Hoenn region, new protagonists, and the legendary Pokémon Groudon and Kyogre.

Generation III added 135 new Pokémon, bringing the overall total to 386. It also introduced Abilities, Pokémon Natures, double battles, Pokémon Contests, Secret Bases, and the ability to choose a male or female player character.

Pokémon Emerald was released later as an enhanced version of Ruby and Sapphire. FireRed and LeafGreen were also Generation III games, but they were remakes of the original Kanto adventures rather than the games that launched the generation. Ruby and Sapphire are therefore the defining starting pair.