Earth-3*
We haven't gotten the full explanation yet, but this is what we know so far:
-Waller was basically shocked by how Syndicate managed to conquer E3, "realizing" there were no stopping the Supers if they ever decided to take over
-We don't know how exactly she managed to return to "our" Earth, but she stated she had a group of "benefactors" that got her back (they may be the shadowy figures from the Dark Crisis epilogue)
-The whereabouts of Black Siren, "Superboy", and verybody else who did the jump to E3 with her, are currently unknown