[B00MER]

This was coded in 2.3.4, unless the variable names have changed this should work in 3.1.3 I hope

<TABLE CELLPADDING=5 CELLSPACING=0 BORDER=0><TR>
{section name=cat_num loop=$categories}
{if $smarty.section.cat_num.index is div by 3}
</TR><TR>
{/if}
<TD>[img]../icon.php?categoryid={$categories[cat_num].categoryid}[/img]</TD>
{/section}
</TR></TABLE>

Change the "is div" by 3 to change up the columns.