The trio of Newton South girls cross country runners marched toward the finish line in unison in Wednesday’s dual-meet finale against Waltham at South High. With rain-soaked uniforms and mud-splattered legs, hardly any room separated the strides of juniors Anna Rhatigan, Lucrecia Aguilar and freshman Rebecca Grusby. It was all by design. That’s what the race plan asked for and the three Lions made sure they followed it to the very last step.