Pick Your 3 Men-Army!

Guts (Berserk) he is really all you need Arima (Tokyo Ghoul) Alucard (Hellsing Ultimate)

I like to imagine that Chizuru Azaiwa (from Squid Girl) could defeat the Colossal Titan (from Attack on Titan), so Chizuru's the only character I'd see as necessary. I guess it wouldn't hurt to also include Gintoki (from Gintama) and Mikasa (from Attack on Titan)