Several interesting answers. Since you just want a border bottom (or top) here are two more. Assuming you want a blue border 3px thick. In the style section you could add
.blueB {background-color:blue; height:3px} or
hr {background-color:blue; color:blue height:3px}
In the table code either
<tr><td colspan='3' class='blueB></td></tr> or
<tr><td colspan='3'><hr></td></tr>