are they only usable in one game?

i have preordered all 3 amiibo of splatoon but i might change my mind as i think the price is not worth if they only works for one single game (pretty sure i will regret anyway)

Getting the squid with the bundle and also pre-ordered the ink girl. That was before knowing what in-game purpose they'd have though.

As far as I know, the amiibos gives you additional challenges and items in Splatoon, and costumes for your Mii fighters in Smash Bros.

I've preordered all 3.

In Splatoon each gives you:

15 challenge missions, unlocking unique clothing (I presume 1 piece per mission, so)
15 unique items of clothing
1 new minigame

Plus the work with Amiibo tap, and for extra lives etc in those games.

I'm mainly getting all 3 because I'm super hyped for Splatoon and I'm looking forward to seeing how they'll be used elsewhere (I'd put money on costumes in Mario Kart for one)

Got the ink girl. I was calling for one forever, and am so glad it`s real. The costume for it is even better x )

I looked for them online and can't find them. I'm going to have to do without it appears.

have the 3 pack ordered :D

nope!

They may only unlock specific content for Splatoon, but will most likely also work as general Amiibo scans in-game, e.g. extra lives in Captain Toad.

I have the triple pack pre-ordered.  I like the way they look!

All 3 as in special pre-order offer!