Question

So if I give Exp share to 5 of my Pokemon in party, not including the lead,
and defeat a haxorus, will all 6 Pokemon in party get 3 attack EV each?

Answer 1

Yep. If they gain experience from a battle, they get EVs.

Comments

So do they all get 3 EVs each?

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Yes, so long as they gained experience points from the defeated Pokémon, in this case three Attack EVs, those EVs will be given out to each Pokemon. None will gain less, none will gain more, unless of course the EVs in the stat are already be maxed out, making those EVs pretty much pointless.