Are they really going to be able to go the distance of the "Dragon Age" in just a trilogy? Depending on what DA media you go with they're only a handful of decades into the century.
And boy they did do a good job of painting themselves into a corner in regards to post-game DLC. Though I'm honestly not surprised - BioWare has never been particularly keen on revealing the ultimate fates of their characters... They just kind of fade away into the background.
My stance above pertained to the idea that a game does not need post-game DLC. Any DLC should add content to a game, right? I mean, that's the point. So if we look at this as we should look at any narrative, content should be meaningful and not superfluous. This was a problem with the ME DLC, and, say, The Darkspawn Chronicles and Leliana's Song in DAO.
If we look at ME2, while the content has been designed to be playable during the actual game, LotSB and Arrival both work better as post-game content, because their content is designed in a way so as to bridge the gap between ME2 and ME3 and make more sense being done in a post suicide mission worldstate. But ME2 isn't so much Shepard's story as the entire trilogy is Shepard's story, so DLC meant to go from point A to point B and progress his/her story in such a way fits.
DA2 itself is Hawke's story, but the game is nothing more than another episode in the life of the overall main character (let's say metacharacter), and that's the 100 years of time that makes up the Dragon Age. So the DLC should, ideally, contribute to both those stories, and so far it has. Whether the DLC should cover beyond the end of the game very much depends on how "Hawke's story" is defined, but I blabbed about that already. It's not like Hawke's ultimate fate can't be revealed come DA3, which from the end of DA2 one could at least expect to have happen.
But I don't see why they couldn't finish out the end of the Dragon Age with a third game. By the end of the Fifth Blight it was already 9:31. DAO encompassed only a year (9:30-9:31) and Awakening was set only six months later and consists of only a very small amount of time. Varric's telling of Hawke's story happens ten years later, so it's 9:40. All the next game has to do is be set during the most important portions of the remaining 60 years, where all the major pieces are on the boards, to finish the tale.
Have they actually said DA is designed as a trilogy?
I believe so, yes. Laidlaw or Gaider somewhere - probably Laidlaw - have stated the third game will be the last "Dragon Age" game, but not the last game set in Thedas.