Why is everyone so eager to believe at this point that Daario slew Brown Ben Plumm somehow? Is it so hard to believe that Plumm switched sides? The last conversation he had with Dany went something like, "Your Grace, you're surrounded on all sides by way too many enemies and can't possibly win... except you have your dragons. You say dragons are out of the question? Then let's loot the city and go. No, not that either? Then you can't win... but you can give me fresh horses and extra bags of gold, and I'll ride nonthreateningly toward your enemy's camp... yeah, that's it."
As for whether this is one of her three betrayals, I doubt it. When a sellsword takes action with "mercenary" reasons, that's just to be expected, really.
I know. I still want to believe that Daario is a good man and had no part in Ben Plumm's 'disappearance', but sellswords are extremely fickle, so who knows what really happened. However it would be just too easy and cliched if Daario does betray Dany (and killing Ben Plumm along the way).