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;}