Edit in JSFiddle

<ul class="tab" id="tab">
    <li>A</li>
    <li>B</li>
    <li>C</li>	
</ul>

<div class="tab_con" id="tab_con">
    <div>1st Contents</div>	
    <div>2nd Contents</div>
    <div>3rd Contents</div>
</div>

* { margin:0; padding:0; }
ul,li { list-style:none; }
a { text-decoration:none; color:#000; }
.tab { border:1px solid #ddd; border-left:none; background:#fff; overflow:hidden; }
.tab li { float:left; width:33.3%; border-left:1px solid #ddd; text-align:center; box-sizing:border-box; }
.tab li { display:inline-block; padding:20px; cursor:pointer; }
.tab li.on { background-color:#eee; color:#f00; }
.tab_con { clear:both; margin-top:5px; border:1px solid #ddd; }
.tab_con div { display:none; height:100px; background:#fff; line-height:100px; text-align:center; }

$(function () {	
	tab('#tab',0);	
});

function tab(e, num){
    var num = num || 0;
    var menu = $(e).children();
    var con = $(e+'_con').children();
    var select = $(menu).eq(num);
    var i = num;

    select.addClass('on');
    con.eq(num).show();

    menu.click(function(){
        if(select!==null){
            select.removeClass("on");
            con.eq(i).hide();
        }

        select = $(this);	
        i = $(this).index();

        select.addClass('on');
        con.eq(i).show();
    });
}