One method would be to use 3 Ditto calls in 3 columns, use the
display parameter to limit the results for each column and use the
start parameter to offset the results for the second and third columns.
<div id="col1">
[[Ditto? &parents=`40` &depth=`1` &display=`4` &tpl=`category`]]
</div>
<div id="col2">
[[Ditto? &parents=`40` &depth=`1` &display=`4` &start=`4` &tpl=`category`]]
</div>
<div id="col3">
[[Ditto? &parents=`40` &depth=`1` &display=`4` &start=`8` &tpl=`category`]]
</div>
You can then use the id of each column to style the contents individually.
Otherwise, you’ll have to use an iteration method:
http://modxcms.com/forums/index.php/topic,15018.0.html
http://modxcms.com/forums/index.php/topic,14478.msg97548.html