Six years ago, the Boston Celtics opened what was supposed to be a three-year championship window after the acquisitions of Kevin Garnett and Ray Allen. Along with Paul Pierce, the new “Big Three” won one title and could have won two more if they had remained healthy. Like NASA getting extended use out of an aging Mars Rover, the Celtics trudged along as contenders for a few more years. The three-year window was open for a fourth year. Then a fifth.