Do you need to send all three event hounds...

Question

To B/W in order to activate the Zoroark event?

Answer 1

I think you just need one, not all three. In the Zoroark event, Zoroark will transform into the beast that is strong against yours (i.e., it'd turn into a Raikou if you had the Suicune). At least, I think so.

Comments

And what if choose Raikou?

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If you choose Raikou, then he'll transform into Entei. It isn't the super effective for all of them, but the other two follow that pattern.

http://www.serebii.net/blackwhite/zoroark.shtml