A lot of forums and sites about Hearthstone mention "3-star masters" decks. I assume it's top level of ranked, but can someone clarify if this true and are there any benefits from being a "3-star master?"
"3 Star Master" medal/rank is the 2nd highest medal/rank in the game, second only to "Grand Master."
As a prefix to a deck build it is used to describe that the deck is suited and tested against very experienced players, meaning a "3 star master" deck build is used for end game rather than early game, mostly because it counters common end game tactics/decks.
Also some deck-builders could use this prefix to imply that you will need good cards or experience to make or use their deck, but it is mostly implied and rarely used.