User blog comment:Miskos3/Fairy Tail Chapter 486: The Fourth Guest, Review/@comment-7418318-20160523164855

Third Seal... hmm. Shall we make a list of possibilities?

Right now I have three.
 * 1) Whatever Irene is doing right now to prepare for fighting Acnologia.
 * 2) Jellal was literally the third seal for Face. I doubt this has any relevance here, but then neither did "five day's worth of food"...
 * 3) Maybe something with Fairy Heart. Cana failed to blast Mavis apart with pure power, so maybe they will switch over to dismantling the crystal bit by bit. If it's made up by several seals layered upon each other, they might reach the "third seal" and thus get close to finishing their job.

That's all the theories I can think of right now. None of them sound too exciting and the latter two are more on the crack-side of things. ^^