CHEYENNE, Wyoming — Three of America’s best-known national parks — Yellowstone, Grand Teton and Great Smoky Mountains — closed their gates Tuesday as parks struggle to keep popular recreation areas open while heeding warnings from officials urging them to prevent spreading the coronavirus at congested sites. Social distancing wasn’t working at Great Smoky Mountains National Park on the border of North Carolina and Tennessee, where about 30,000 people lured by good weather, wildflowers and spring break visited each day last week, park officials said.