@media (min-width: 319px) {

  .center, .right { 
    float: none;
    width: auto;
  }
  
}

@media (min-width: 768px) {

  .center { 
    float: left;
    width: 70%;
  }
  
  .right {
    float: right;
    width: 20%;
  }

}