DA3 should be fresh and unclutted by past choices, seeing how the game takes place 10 years or so after DA2. Most if not all choices you made would be old news and would not affect the story in DA3 one bit, seeing how it will be in a different place than Kirkwall or Ferelden so those choices you made would never be seen in Orlais. I am thinking the only choices that mattered would involve the mage/templar conflict and the old companions you have in your pary from DAO, DAA & DA2.
Well, there was really no world-shattering choices in DA2, other than whom did Hawke select - templars or mages - and thath wasn't even that important, as no matter what, you killed Orsino, Meredith, and Chantry has fallen.
However, there were quite important choices in Origins that could affect Orlais, if that is where the game is heading. For one, the monarch. Who is the ruler of Ferelden could have tremendous repercussions, like unhardened Alistair could be a puppet and let Ferelden fall to Orlais, hardened Alistair may start a war with them, a single Alistair could marry the Empress, and so on. Other choices like who is the king of the dwarves - whether they'd be at a cultural renaissance due to Bhelen and the use of the castless to fight off darkspawn, or maybe they lose more and more to the taint with a weak king such as Harrowmont.
Then there is the business of the Circle. Has it been cleansed and mages hate templars a little more, or has it been saved and mages would be willing to cooperate more eagerly?
Also, the werewolves and Dalish. The Origin's clan could be very important to Dalish in general; are they willing to forgive the humans, or are they even more xenophobic?