After the scintillating highs and surprises of its second season, "Boardwalk Empire" returns following a passage of time, which can always be a trifle unsettling. In short order, though, the program finds its groove, with new rivals, shifting relationships and strained allegiances, all set against the intoxicating backdrop of the booze-running 1920s and wildly corrupt Harding administration. Filled with first-rate performances, jarring violence and esoteric sexual acts, the show ranks alongside "Game of Thrones" as HBO's twin titans, with prodigious strengths that far outweigh a few lingering, far-from-fatal weaknesses.