You can only find them in Sapphire right? If so, how can I get them and what do I need to do to get them

First off you'll need to understand braille. Second, you'll need a Pokemon who knows dig, a Relicanth, and a Wailord. Third, you'll need to read and understand complex conveyor belt patterns.

Oi. Could I do that after I beat the Elite Four? Or do I have to do it before?

you can get them in ruby sapphire emerald and platinum and maybe hg/ss

oh lordy. It's been so long since I caught those guys. I don't remember the specifics.

But here's a page on them:

Regi Trio

That should tell you how to get them. I don't remember them being all that difficult.

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