Solved it, that was indeed it. Reduced the padding in the listboxContents div, and that enabled that third category to get onto the same row.

I would still love to know where I could adjust the
<div class="categoryListBoxContents" style="width:33%;">

so that it would read

<div class="categoryListBoxContents" style="width:31%;">

Any takers?
thanks all,

JSC