Set float = left for first 2 DIVs and for third set margin-left = (width of DIV1) + (width of DIV2)
<div style = "background-color:Red;height:50px;width:100px;float:left"></div>
<div style = "background-color:Blue;height:50px;width:100px;float:left"></div>
<div style = "background-color:Green;height:50px;width:100px;margin-left:200px"></div>
Demo