I'm not sure, what's your friend code? I can check it right away. 
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Pokécharms
Hollow Omega
Gio20400
0748-6993-4251
Hollow Omega
Alright, I added you!
Gio20400
Thanks! 
Btw I think you'll like what I have in my Safari
Btw I think you'll like what I have in my Safari
Hollow Omega
I'm looking forward to finding out.
Gio20400
DUDE
YOU HAVE MACHOKE AND PANCHAM IN YOUR SAFARI
WHAT'S IN THE THIRD SLOT?
YOU HAVE MACHOKE AND PANCHAM IN YOUR SAFARI
WHAT'S IN THE THIRD SLOT?
Hollow Omega
No idea, you also have two Pokémon. Have you beaten the Elite four?
Gio20400
Yes I have
Did you beat the Elite 4 too?
Did you beat the Elite 4 too?
Hollow Omega
I did!
Gio20400
I think we have to be online at the same time to get the third slot
Hollow Omega
That could be it, I'm not 100% sure how it works.
Gio20400
"...with the third Pokémon slot appearing should the person have defeated the Elite Four, with them getting Hidden Abilities if the player has been online at the same time as you"
Found this on Serebii
Found this on Serebii
Hollow Omega
Looks like you're right then, I'm currently in Pokémon X and I should be online, if you want to unlock the third slot
Gio20400
You're not online now ^^;
Hollow Omega
Really? That's weird, lemme change that.
Gio20400
You're on now XD