Leliana shows up in the third game regardless of what happens to her in DAO. I don't think it's possible to kill her in Origins but even if you can the series overwrites that choice when you play Inquisition. Unfortunately she's not playable in Inquisition and only an adviser. She's not in DA2 to my recollection though maybe she's mentioned in the codex or something.

After my fling with Morrigan I settled down with Leliana in Origins. It's pretty cool in Inquisition when she refers to the Hero of Ferelden as her love. Still together after 10 years!