She's dead, as well as Iorveth, Skaggs, Adda, Anais, etc.
Any special character not mentioned in TW3 is presumed dead.
Not necessarily.
There is a dialogue with Philippa in TW3 about Saskia.
However I was not too happy with it. Because in my TW2 game, I cured Saskia and let her live. My favourite character after all. And I imported this save. But in the dialogue with Philippa, Geralt actually asks Philippa if Saskia managed to get herself free of Philippa's spell. Dammit he should not ask that when he was the one who cured that spell.
Then Philippa says she lost control of Saskia when the war broke out, and does not know where she is. She is most likely alive though.
So on one hand, I was at least glad to see a reference to her.
On the other, I was sad the reference ignored that I cured her.