html

<div class=mod clearfix>                
          <div class=ml >          
              <p>左侧定宽</p>         
          </div>
          <div class=mc >          
              <p>中间定宽</p>         
          </div>       
          <div class=mr>
              <div class=modcon>
                   <p>右侧自适应</p>
              </div>
          </div>               
      </div>

css

.ml{float:left; width:190px;}
.mc{float:left; width:190px;margin-left:10px;}
.mr{}
.mr .modcon{margin-left:400px;}