They are all different types of games, different types of RPGs. I'll clarify now that I have 750 hours in Skyrim, around 50 hours in Oblivion and 30 in Morrowind (I know, barely anything), I played a little over 40 hours of The Witcher before the SSD it was installed on crapped out causing me to lose my saves, I beat The Witcher 2 twice going down each of the two paths, I beat Dragon Age: Origins 4 times as 4 different characters, I beat Dragon Age 2 twice (sided with mages both times though), I beat Dragon Age: Inquisition once (a 150 hour playthrough). I will start playing The Witcher 3 in a few days.
The Witcher games and Dragon Age cross paths more than either of them do with Skyrim. Both have role-playing within a story, a choice-consequence narrative design. Dragon Age far exceeds The Witcher in this regard purely in terms of role-playing; Dragon Age has far more choices, far more consequences, and the results of your choices have more than one impact (unforeseen consequences, chain reactions, more than just 2 outcomes for every major quest).
Remember how I said I played "the two paths" in The Witcher 2? Dragon Age: Origins and Inquisition have far more than just two paths; the plot and character development can change completely depending on your actions, to a much greater extent than the first two Witcher games. Save game importing between the first two Witcher games is also extremely insignificant, changing only two minor scenes (a brief dialogue option and one insignificant encounter). Plus, you're stuck as playing a predefined character in The Witcher which inherently limits its role playing (he is far more defined at the start than say, Commander Shepard, who is a clean slate since the game lets you choose his background). While The Witcher games have much less role-playing, the writing quality is at least slightly better. Skyrim doesn't really have this kind of role-playing at all, neither do the other Elder Scrolls games.
Skyrim, like its predecessors, is more about freeplay. You can role-play as any type of character pretty much; a petty roadside bandit, a family man, a scholar, etc. You can join different factions, going down different major quest lines. But it doesn't have much of a story. It has the most freedom by far, it's the only true open world game between it, Dragon Age: Inquisition, and The Witcher 3.
In terms of gameplay mechanics, combat, and statistics, The Witcher games again provide the least. There is only one "class" or playstyle (sword + 5 or 6 spells), and the second game has some of the most simplified hack and slash combat. Leveling up is rather insignificant in the first two The Witcher games since there are very few stats, only a basic persuasion skill and not many new combat abilities.
Skyrim on the other hand has many different skills and over 20 different playstyles. It has the most variety, while Dragon Age is the most tactical. The three Dragon Age games (especially the first and third) have pause-and-play tactical combat, in which you utilize a party of four characters. You can take control of each character or just issue commands, and in Origins you can program their AI to respond specifically to different combat scenarios. Dragon Age has a few playstyles under three basic classes; Warrior, Rogue, Mage. Each one has specializations that can almost be compared to prestige classes in D&D (but a lot more simple).
The first two Witcher games are extremely linear, 3 is open world to some degree (might be region locked, I'm not sure). Dragon Age: Origins is hub-based like the D&D classics, Dragon Age 2 takes place within a small city and doesn't really branch out much, Inquisition is like Origins times fifty in terms of scale; still hub-based but the non-hub maps are gigantic (more than a few are the size of entire open world games).
TLR: They are different kinds of RPGs. Skyrim is not story focused like the other two, it's a freeplay RPG and the only true open world one. Dragon Age and The Witcher are both story focused yet The Witcher remains an action RPG unlike Dragon Age, and The Witcher has significantly less role playing but slightly better writing quality in comparison.