BEIJING—Kaillie Humphries crossed the finish line, jumped from her sled and hoisted an American flag that someone had just handed her into the frosty air. “U-S-A! U-S-A!” she chanted. Four years ago, nobody would have seen that coming. The former Canadian bobsled heroine is an Olympic gold medalist for the third time—and for the first time, as a U.S. citizen. Humphries is monobob’s first Olympic champion, finishing off a surprisingly easy win at the Beijing Games on Monday. She completed four runs in 4 minutes, 19.27 seconds to lead a gold-silver finish for the U.S. women’s program, with teammate Elana Meyers Taylor placing second. “As I age, I’ve learned life is very unpredictable and we don’t always know our path forward and what the future holds,” Humphries said. “We do the best we can with each and every obstacle or path change that gets thrown at us. What’s important is …