The pretty prince Alistair from Dragon Age: Origins, who also made a cameo in Dragon Age 2, won’t be available for your party in Dragon Age: Inquisition. This was confirmed by Bioware’s Mike Darrah on Twitter, who also added “You’ll have to wait and see on the rest” regarding whether Alistair would make an appearance.
As one of the core characters of Dragon Age, who appeared since the very beginning, Alistair played a pivotal role in carrying out events throughout the first game, all the way up to the battle with the Archdemon. His cameo in Dragon Age 2 involved nothing more than meeting and greeting protagonist Hawke. As for what kind of role he could play in Inquisition, it simply remains to be seen.
You can import your previous games’ saves though, which will determine if Alistair even appears at all. Dragon Age: Inquisition is currently set to release for Xbox 360, PS3, PS4, Xbox One and PC.