All three Pokemon cannot be found via Dex-Nav
There are only a few Pokemon which can be searched with a Dex-Nav.
See here for the list
But there are ways to get HAs on them without the Dex-Nav
Greninja - Frogadier is Greninja's pre-evolution and can be caught with Protean in a Water type Friend Safari. The transfer them to ORAS via PokeBank
Gliscor : In XY Gliagars can be found in hordes on Route 19. Grab an Oddish or a Sweet-Scenter and find hordes of Gligars. Test to see their ability by using Toxic or a Trace Ability. Then transfer to ORAS via Pokebank
Snorlax : In BW2 a girl called Yancy gives you a HA Snorlax transfer it over to XY via Poke-Transporter and then over to ORAS with PokeBank.
See this to get to a sidequest then watch this
If you can't do these things then try the GTS or ask around.in Trading Forums or the DB Chat. When on GTS don't be fooled by idiots who lie about stuff though
Good Luck :D