Flexbox

.flex
   {
      /* basic styling */
      width: 350px;
      height: 200px;
      border: 1px solid #555;
      font: 14px Arial;
 
      /* flexbox setup */
      display: -webkit-flex;
      -webkit-flex-direction: row;
 
      display: flex;
      flex-direction: row;
   }
 
   .flex > div
   {
      -webkit-flex: 1 1 auto;
      flex: 1 1 auto;
 
      width: 30px; /* To make the transition work nicely.  (Transitions to/from
                      "width:auto" are buggy in Gecko and Webkit, at least.
                      See http://bugzil.la/731886 for more info.) */
 
      -webkit-transition: width 0.7s ease-out;
      transition: width 0.7s ease-out;
   }
 
   /* colors */
   .flex > div:nth-child(1){ background : #009246; }
   .flex > div:nth-child(2){ background : #F1F2F1; }
   .flex > div:nth-child(3){ background : #CE2B37; }
 
   .flex > div:hover
   {
        width: 200px;
   }

uno

due

tre