People are speculating that the thrid treason could be by Missandei. The show stated her as Dany's "most trusted advisor" (which sounded suspicious) and Davos seems dubious of her past.
I don’t know why Missandei being Dany’s most trusted advisor would sound suspicious. I mean, she is Dany’s most trusted advisor and confidant. Also, what purpose would it serve to have Missandei betray Dany? What reason could she have to do that?
There needs to be a narrative purpose for such an action and at the moment I can’t think of one, maybe that will change, but as of right now, I don’t see it.