Avengers/guardians of the galaxy
 

It's possible the avengers team-up with guardians of the galaxy in the third installment ?

Chris Pratt did sign a three-picture deal. Since Guardians of the Galaxy is confirmed to get a sequel, at least Star-Lord might show up in Avengers 3. Especially if the Infinity Gauntlet is involved.

Guardians of the Galaxy is definitely in the same movie universe as The Avengers, but there are no announced crossovers yet. The Guardians are confirmed to have no connection to The Avengers: Age of Ultron, and I haven't heard anything about a Guardians of the Galaxy sequel, though that could change when the movie actually comes out. I think it's fair to assume that they'll have some sort of connection to The Avengers 3, even if there isn't much basis for it