Keep reading the Manga after about 10-15 more Chapters Sasuke will complete his Susano'o
Sasuke's Susano'o was already complete vs. Danzo. He just doesn't have the items that Itachi had. They only exist within the user himself which means they probably died with Itachi.
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3rd legendary item could be the "Magatama" jewel necklace. It's in the Japanese lore as one of the 3 legendary items with Yata's Mirror and Blade of Kusanagi. Kishi subtituted the blade of Kusanagi with the Totsuka sword that Itachi had but it followed the lore.
Basically in the lore, Amaterasu goes into hiding from Susano'o and plunges the world into darkness. Susano'o lures her out with Yata's Mirror and Magatama necklace. He later slays the 8-headed snake (Orochi) using the blade of Totsuka and gets the Blade of Kusanagi from one of its heads. He gives the Blade of Kusanagi, Yata's Mirror and Magatama necklace to Amaterasu as a token of apology.
In short form; Sasuke's Susano'o is complete. 3rd Item is called Magatama necklace and Yata's Mirror and Blade of Totsuka are spirit items that exist within its user. Itachi died with it and we haven't seen Sasuke with either item even though he mass spammed his Susano'o.