First, let's recap some of the notable special artifacts introduced so far in the Marvel Cinematic Universe and where they are currently located:
- Space Stone - Blue in color, the Tesseract (Cosmic Cube) is currently located on Asgard where it was returned for safekeeping after the events of The Avengers.
- Mind Stone - Yellow in color, the Mind Stone was weaponized into a sceptre and given to Loki by Thanos to aid in his failed attack on Earth. It is later acquired by Hydra during the fall of S.H.I.E.L.D. and used by Ultron in the creation of of the new Avenger known as Vision where it currently sits attached to his forehead.
- Reality Stone - Red in color, the Reality Stone is actually a liquid that Malekith sought out to attack Asgard and Earth in Thor: The Dark World. It was recovered and given to The Collector to protect on the space station Knowhere.
- Power Stone - Purple in color, this stone was housed inside the Orb recovered by Peter Quill in Guardians of the Galaxy. After the Guardians recovered the Stone from Ronan they let the Nova Corps on the planet Xandar keep it in their vault.
The two remaining stones are the Time Stone and Soul Stone and it's the latter that was carried by Doctor Strange in the Marvel Comics which makes the idea of the Time Stone possibly being in that movie different. If anything, the time-travel nature of Guardians of the Galaxy stories from the comics make its sequel the logical place to utilize the Time Stone. Of course, given the time-travel nature of the stone itself, there are creative ways to have it appear in more than one movie.
We do know that these last two Stones will be introduced within the next three years before Avengers: Infinity War begins in summer 2018. When it does, big bad villain Thanos will retrieve and assemble them all into the Infinity Gauntlet. And unlike the comics, there are two of magical gloves in the MCU.
- Left Gauntlet - on Thanos' ship, equipped during the post-credits button scene of Avengers: Age of Ultron
- Right Gauntlet - in Odin's vault on Asgard as seen in Thor
From the teaser footage of Phase 3 (image up top), it's the left gauntlet that Thanos will use with all six gems inside them - the one he inexplicably already has. If we go by the image up top, there's the green stone and the larger, heavenly looking one that goes on the back of the glove. The green one could very well be the stone inside the Eye of Agamotto. It's either that or this is all just a big coincidence and Kevin Feige is intentionally throwing fans for a spin before the film releases.