The Windrunners could a red herring. Hear me out.
Il'gynoth never specified what he meant by "her third death." We've just ASSUMED it has to be a female character who dies/has died three times. This could be many things.
>Jaina
When Daelin finds her at Theramore during the Rexxar campaign, he mentions how he thought she'd died in Lordaeron. After the Bombing, the peace-loving woman she HAD been died along with its victims. And it seems like her mother has some harsh words for her in Boralus, possibly even disowning her - or, in other words, Katherine might say that Jaina is "dead to her." Three deaths.
>Alexstrasza
She knows Malygos is dead. She killed him herself. She knows Neltharion is dead - she helped kill him herself. What if she never reacted to Ysera's death because she DOES NOT KNOW that her clutch-sister is dead? And when she is shown a vision of it, she will have WITNESSED the third death of another Aspect. A stretch, but it could match "her third death" in the same way you might say a young child sees their first death.
"Her" could also refer to an institution, or a member of it. Which leads to...
>Calia
Her father is dead, which ended her kingdom. Her brother is dead, which ended his reign. If she were to perish, it would end her line... and perhaps usher in the coming of the Old Gods.
But if we go back to the Windrunners, Sylvanas becomes rather likely again! But why, exactly? Maybe it's not the hour of her third death, but the hour of her third Death. Capital-D Death, the personification.
>Sylvanas
The Lich King, who forced her into servitude for a time, has been called a god of Death by the Vrykul. Yogg-Saron may have tormented her in the Void (and called himself the God of Death), but she has never served him - which is why he does not count. Helya, however, DOES. A goddess of Death, with whom Sylvanas made a deal that presumably involved serving her in some way. So who is Sylvanas' third Death?
Bwonsamdi, the Loa of Death.