2 EMPLOYEES WERE CHANGING BUCKET ON CAT BACKHOE 426B. A THIRD EMPLOYEE APPROACHED WORK SCENE AND IN OPERATION. THE SECOND EMPLOYEE WAS SEARCHING FOR PLUGS IN THE CAB AND ACCIDENTLY, ENGAGED THE BOOM SWING LEVER. THE 3RD EMPLOYEE MOVED BACKWARD AS THE BOOM CAUSE FORWARD PUSHED HIM ALSO STRIPPED OVER A CUTTING EDGE LYING ON THE GROUND EMPLOYEE FELL ON HIS HIP.