The easiest solution is this (in the HTML editor, of course):
<div style="width:246px;margin-top:11px;margin-bottom:27px;background-color:#959EA1;padding:18px 0 0 18px;">
CODE OF ALL THREE IMAGES HERE
<br style="clear:both;" />
But don't forget to replace aligncenter and alignright in the second and third image codes with alignleft.
You can change the margin-top and margin-bottom numbers to adjust the space that separates the box from the header and the footer of the theme.
959EA1 is the color of the title of your page. But you can change it - palettes and color codes here:
If you don't like the result (for instance, if you'd like to change the distance between the images), we'll need to create a more complicated code.