This requires that you have the World Pokedex, which allows you to travel between regions. You just take the plane to Hoenn and Fly to Sootopolis City, then Dive just outside the volcano. You ought to find a Relicanth in the seaweed, but it might take a while, since they're fairly rare and annoying to catch.
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You can catch it when you use dive.
You need relicanth and wailord to catch the regis in sapphire/ruby/emerald. You cannot catch the three regis in platinum unless you have the event Regigigas (this event has now ended). If you have the special regigigas in your party, three new caves will open up in Sinnoh and you will be able to catch the three regis.
You find relicanth underwater near sootopolis city.
how do you catch a mewtwo on Pokemon platinum
you cannot catch the 3 regi's in Pokemon platinum you have to catch them in Pokemon emerald then transfer them to Pokemon platinum. thnx