They’re not dissing Third Avenue anymore. Reflecting the corridor’s rising fortunes after years of “Third’s the Bird” jokes, the redevelopment of 850 Third has added a tasty side dish to its $80 million menu of inside-and-out repositioning.  Waterman Interests and HPS Investment Partners, owners of the mostly vacant, 605,000 square-foot Midcentury tower between East 51st and 52nd streets, signed a big-league restaurant even as work continues apace to re-launch the refreshed property by the end of the year. The rebirth of 850 Third comes after companies left the avenue for more modern and glamorous locations on Park Avenue, Sixth Avenue and the far West Side. The avenue’s recent resurgence includes huge conversion projects that are eliminating obsolescent offices and will soon draw more residents. Kellari Hospitality Group will launch a 4,500 square-foot Greek and Mediterranean eatery called Artemis on the ground floor of 850 Third Ave.…