Well we know during the 2nd Curse that Zelena manipulated Snowing's curse so why not again right?
So I think Emma manipulated the 3rd Curse for her own liking but WHO cast the curse?
Everyone of the characters are still there except.....*drumroll*
LANCELOT
What if Gwen cast the curse using Lancelot's heart? The pnly question is why
It was probably Artur's doing
ANOTHER THEORY:
Is that Zelena cast the 3rd curse i mean.....
Look at this... Regina cast the 1st curse because Snowing Snowing cast the 2nd curse because of Zelena Then the 3rd one... Zelena cast the 3rd curse because of...???
But what possible heart could have been used? HER BABY... I think the baby is already dead poisoning her body and Zelena is the possible death in Swan Song :p