Three American prisoners freed by North Korea were flying back to the United States on Wednesday, heading toward an air base where President Trump planned to personally welcome them home as he celebrates a diplomatic breakthrough in advance of nuclear talks. The three prisoners, all American citizens of Korean descent, were described as healthy and able to walk on their own after months of imprisonment by one of the world’s most repressive governments. They were scheduled to land at 2 a.m. Thursday at Joint Base Andrews outside Washington, where Mr. Trump intended to greet them.