Hello, everyone.
There are many differences between those games - setting, combat mechanics, theme&mood, party companions (or lack of them), and so on.
What I find the most interesting difference is that DA:I is meant to be a role-playing campaign about being a head (or at least a figurehead) of certain faction in the world of Thedas, (that is, titular Inquisition). Whereas Skyrim turns out to have been designed as a campaign for a classic rpg adventurer archetype, or actually a not-so-narrow variety of archetypes, while maintaining a classless PC game mechanics; and that is also interesting.
The Witcher 3, sadly delayed again, is entirely different variety, because it is based on a book series. Like "Betrayal of Krondor", for which you can easily find which of the R.Feist novel(s) it draws from, the Witcher game series draws from Witcher book series; Witcher 1 from early books (striga, rose order), Witcher 2 from mid-books (with its climax drawing from the Thanedd book events), and Witcher 3 seems to be drawing most from the later books in those series (Geralt + Yen,Ciri; emperor Emhyr presence).