Yeah I don't remember what each items specifically is. But I believe that Itachi only had two out of three of them. The necklace and mirror thingy perhaps? I bet you Madara has the other one. What ever it is. OH! What if Pain had it on the Deva Path? Maybe. O.O
he only showed 2 of the 3. the missing one is this necklace that looks like the sage of 6 paths' necklace
which also looks like naruto's new fox mode necklace
Didn't Itachi have all 3 pieces of this Japanese Mythology when he was fighting Sasuke which took place AFTER he met up with Naruto and gave him some of *his* power.
Actually Itachi showed only one of those items, as did Orochimaru. If you would look in
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you will see that the imperial regalia are the following objects:
1) Kusanagi (grass cutter) sword, owned and used by Orochimaru in his fight vs the 3rd hokage, vs 4-tailed kyuubi in the beginning of shippuuden and vs Itachi at the end of the Sasuke vs Itachi fight. Currently sealed away by the Totsuka sword (it disappeared when Itachi sealed Orochimaru so I presume it is in there).
2)Yata no Kagami (Yata's mirror) is the shield that reflects any attack, it was in Itachi's possession during the Sasuke vs Itachi fight. Current owner unknown.
3)Yasakani no Magatama (Yakasani jewel) "is believed to be a necklace composed of jade magatama stones instead of a solitary gem as depicted in popular culture." (in Japanese mythology, text is copied from wikipedia). It has yet to be shown in the Naruto manga.
So Itachi only has one of the imperial regalia items or at least only one is shown to belong to him up until now. There is the possibility he has the necklace but that is yet to be established.