A few possible options I see here as someone that might have their "third death" story-wise:
1. Alleria
As the OP shows, Alleria considers herself to have died twice. Blizzard saying this seems like a highly ham-fisted way of making her the third death, so it would make sense. The problem I see is that while she may have considered herself twice dead, she never actually died. She has stayed Alleria, even if she has been taken in by the Void. For an Old God to have this kind of insight to her personality on a distant world would feel weird. Then again, Blizz did try to ham-fist that quote in, so...
2. Sylvanas
She seems to be a main focus in this expansion and may well be a raid boss at the end of it. Even the most flattering stories for her at this point seem to end in some sort of heroic death rather than living with everyone holding hands. The main problem here is that she's already died three times (fighting Arthas, jumping off ICC, shot by Godfrey), but Blizz could easily explain one away if they so choose. Maybe the ICC one wasn't an actual death, but rather a Val'kyr trick to get a new boss. Maybe the Arthas one doesn't technically count as Sylvanas really became a different person after Arthas raised her. Maybe she was just really sleepy rather than dead from Godfrey (OK, that one's a stretch).
3. Azeroth
We've seen two possible "deaths" for Azeroth in the form of the Sundering and the Shattering. This expansion seems like she's at risk for a third death, might she be the choice? Interesting thought on this one: Wrathion's quote upon eating the Thunder King's heart. "We must rebuild the Final Titan!" How does one rebuild a Titan persay? What exactly does Reorigination do to Azeroth herself? I almost wonder if such events actually somewhat kill her down to a minor cell or two where she has to regrow/be reborn. If so, she's a prime candidate here, particularly as her death would likely break down a lot of safeguards on the Old Gods.
4. Helya
We killed her once in the ToV raid and fought her intensely in Maw of Souls. You could definitely argue that what Odyn did to her was a death in and of itself. And yet, one of the island expeditions gives us evidence that Helya yet lives. She seems like a good candidate for a third death as well, especially given whatever her deal with Sylvanas was and her control over the Shadowlands.