“We’re getting married!” This rather inevitable line crops up early in “My Big Fat Greek Wedding 3,” and if you’re like me, it will inspire mixed reactions. First: Wait, so soon? We didn’t know anyone was even engaged! And second: Phew, it’s about time! Because, just like there can be no sunrise over the glittering Ionian sea without a sun, there can be no “big fat Greek wedding” movie without ... you know.