.caja 
  { 
    float:left; 
  }
  .caja1 
  { 
    float:left;
    margin-left:20px; 
    margin-bottom: 20px; 
  }  
  .caja2 
  { 
    float:left; 
    margin-left:20px; 
    margin-right: 20px;
    margin-bottom: 20px; 
  }

  .caja3 
  {
    float:left; 
    margin-top: 25px;
    margin-bottom: 40px;
    margin-right: 20px;
    margin-left:20px; 
  }