Output of resources in rows something like outputting 4 resources: <row><resource><resource><resource></row> <row><resource></row>

bootstrap_output_in_rows

<!-- outputting in rows -->

<div class="row">
[[pdoResources?
 
   &parents=`[[*id]]`
   &depth=`2`
   &limit=`99`

   &includeTVList=`myTv,anotherTv`

   &tpl=`my.chunk`

]]

</div>

my.chunk

<div class="col-md-4 ">
do something
</div>

[[+idx:if=`[[+idx:mod=`3`]]`:eq=`0`:then=`</div><br /><div class="row">`:else=``]]

Better use &tpl=my.chunk &tpl_n4=my.4th.chunk`` for that

See: http://docs.modx.pro/en/components/pdotools/general-settings

This is shorter and one parsing loop less (but it does not work in pdoTools fastmode):

[[+idx:mod=`3`:eq=`0`:then=`</div><br /><div class="row">`:else=``]]

For fast mode try this (untested):

[[+idx:if=`[[+idx]]`:mod=`3`:eq=`0`:then=`</div><br /><div class="row">`:else=``]]