By Yereth Rosen ANCHORAGE, Alaska (Reuters) - U.S. Senator Lisa Murkowski of Alaska publicly backed gay marriage on Wednesday, becoming the third Republican senator to do so as she spoke out ahead of potentially landmark rulings from the Supreme Court on the issue. Murkowski, 56, wrote in an essay posted on her Senate website that her decision was swayed, in part, by meeting a lesbian couple from Anchorage, one of whom was in the National Guard, who had adopted four children. "This first-class Alaskan family still lives a second-class existence," Murkowski said in her essay. ...