The figures of Susano'o in the manga shows 3 arms however it is kinda difficult to spot. Anyway you can check it in naruto.wikia.com and look for Susano'o. It is stated there that a 3rd arm grows from his right arm. Weird eh? Now, I remember the name of the celestial mirror, its the Yata No Kagami (Eight Span Mirror) on the left hand, the sake jar on the right, plus the Sword Of Totsuka on the additional right arm...:naruto:
