You should use CSS to align the textbox. The reason your code above does not work is because by default a div's width is the same as the container it's in, therefore in your example it is pushed below.
The following would work.
<td colspan="2" class="cell">
<asp:Label ID="Label6" runat="server" Text="Label"></asp:Label>
<asp:TextBox ID="TextBox3" runat="server" CssClass="righttextbox"></asp:TextBox>
</td>
In your CSS file:
.cell
{
text-align:left;
}
.righttextbox
{
float:right;
}