Question

How do you get Genesect, Keldeo, and Meloetta in ORAS?

Answer 1

Genesect, Keldeo, and Meloetta are Event only Pokemon from Generation V. They were released as movie promo's, or at special events. I don't think these event Pokemon have been released for Generation VI yet so you will have to migrate them from a Gen V game (white, black) or find someone else who has done so and trade for it.

This article in bulbapedia explains a bit more about the event Pokemon.

Answer 2

There isn't an In-Game way to get any of the 3 Pokemon. You'll have to Hack or Trade.

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Hack

You can just Inject the 3 Pokemon into your Game. There is a chance that your Save File might corrupt so keep that risk in mind. The most easy way of hacking would be via Powersave or QR injecting

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Trade

Although most Keldeos, Meloettas and Genesects are likely to be hacked you can still get a legitimate one as there were Events for all 3 in 2012 - 2013
Recieving a Hacked Pokemon via trade is a lot less risky than hacking into the game itself.

Comments

Great,  there isn't a way in-game . Both options aren't applicable . I was hoping for a meloetta ....

Answer 3

As far as I know you cannot get them in ORAS. Unless there is an event in the future or you use Poketransfer to transfer to transfer them, you cannot get them without hacking (not that I suggest it)
List of legendary Pokemon you can catch in ORAS without events

Comments

Oh yeah, trading too.
Source: the answer below. :)