Hi,
I've been pondering this one and I think I have come up with a solution for you, or at least a starting point.
The Feed Manager will wrap the feeds in a div with a class of "rssentries" and then each feed entry will be wrapped in a div with a class of "rssfeedentryTrue". So, the generated markup will be similar to:
<div class="rssentries">
<div class="rssfeedentryTrue">
<div class="rsstitle">Feed Entry Title</div>
<div class="rsstext">Blah blah blah....Feed text....Blah blah blah</div>
</div>
</div>
With this markup in mind, it is fairly straight forward to make the entries show in a horizontal list with CSS. Here's an example:
.rssentries{float: left; width: 400px;}
.rssfeedentryTrue{float: left; width: 33%; margin-right: 10px;}
Finally, due to the use of the float, you will want to clear the float. At the very bottom of the Feed Manager module is an empty div with the .cleared class to help with this. You need to make sure you have the .cleared class in your css with a clear: both; rule to clear the floats. If you don't want to set it for your entire skin, create a selector and rule combo like this:
.rssfeedmodule .cleared{clear: left;}
HTH,
Joe D.