One example, not realy UDT:

Data:
<?xml version="1.0" encoding="utf-8" ?>
<items>
  <item>A</item>
  <item>B</item>
  <item>C</item>
  <item>D</item>
  <item>E</item>
  <item>F</item>
</items>

Script:
<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:output method="html" indent="yes"/>
  <xsl:template match="items">
    <table>
      <xsl:for-each select ="item[position() mod 3 = 1]" >
        <xsl:variable name ="row" select ="position()"/>
        <tr>
          <td>
            <xsl:value-of select ="."/>
          </td>
          <td>
            <xsl:value-of select ="/items/item[($row -1)*3 +2]"/>
          </td>
          <td>
            <xsl:value-of select ="//item[($row -1)*3 +3]"/>
          </td>
        </tr>
      </xsl:for-each>
    </table>
  </xsl:template>
</xsl:stylesheet>

Returns

A B C
D E F