...if not all dlc are coming to ps3...

I try to explain my thoughts, I'm not an english teacher..

I don't know much about Dawnguard, I still hope we'll get it one day, but I'm pretty sure that it is located in Skyrim, it adds new stuffs and new events in Skyrim, right?
Dawnguard adds something else to a world that ps3 is still not able to manage very well (Sony or Bethesda fault, it doesn't matter now...)
Dragonborn, on the other side, is located in another place, Solstheim, an island faraway from Skyrim and much smaller...there will be limitated consequences on Skyrim life and interactions with the world and other quests, and so, probably, while you are playng in Solstheim, the ps3 system can forget about the rest of the world, and it would be easier to make the ps3 ram handle it.
I mean that also if Dragonborn is bigger than Dawnguard it would be less demanding for the system and is what we'll get on ps3.
Is my argument valid?

As far as I know, the answer to your question is "yes and no."
Yes, that it is in "another world" so the console wouldn't have to be worried about what's going on in Whiterun, or the "Skyrim NPC's" business on top of what's going on in that world. Most likely it would perform the same.
No that it does add to the overall memory file, which breaks the game.

I think that folks are right that claim Hearthfire and Dawnguard will completely break the game. It adds more on top of the general goings on in Skryim that has to be kept track of. Keeping track of the houses one can build and the people that live there as well as the reported vampire attacks changing the dynamics of a town or city would make Skyrim unplayable. Adding on top of that the new quests, caves, and whatever...well, that would be the end of it.