Определение третьего уровня вложенности в меню.

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE xsl:stylesheet>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
   <xsl:output xmlns="http://www.w3.org/TR/xhtml1/strict" doctype-public="-//W3C//DTD XHTML 1.0 Strict//EN" encoding="utf-8" indent="yes" method="html" omit-xml-declaration="no" version="1.0" media-type="text/xml"/>
   
   <xsl:template match="/">
      <xsl:apply-templates select="shop"/>
   </xsl:template>
   <!-- Шаблон для магазина -->
   <xsl:template match="/shop">
      <SCRIPT>
         <xsl:comment>
            <xsl:text disable-output-escaping="yes">
               <![CDATA[
               function show_hide_menu(id)
               {
               if (obj = document.getElementById(id))
               {
               obj.style.display == 'none' ? obj.style.display = 'block' : obj.style.display = 'none';
               return false;
               }
               return true;
               }
               function active(){
               var obj = document.getElementById('active');
               while (obj){
               obj = obj.parentNode;
               if(obj.tagName=='UL'){
               obj.style.display='block';
               act = obj.parentNode;
               }
               }
               }
               ]]>
            </xsl:text>
         </xsl:comment>
      </SCRIPT>
      <xsl:variable name="parent_group_id" select="@current_group_id"/>
      <!-- Выводим группы магазина -->
      <ul class="left_menu">
         <xsl:apply-templates select="//group[@parent=0]"/>
      </ul>
   </xsl:template>
   <!-- Шаблон для групп товара -->
   <xsl:template match="group">
      <xsl:variable name="current_group_id" select="/shop/ТекущаяГруппа"/>
      <li>
         <xsl:if test="$current_group_id = @id">
            <xsl:attribute name="id">active</xsl:attribute>
         </xsl:if>
         <xsl:choose>
            <xsl:when test="$current_group_id = @id or count(.//group[@id=$current_group_id])=1">
               <!-- Определяем стиль вывода ссылки -->
               <xsl:variable name="link_style">
                  <xsl:choose>
                     <!-- Выделяем текущую страницу жирным (если это текущая страница) -->
                     <xsl:when test="$current_group_id = @id">font-weight: bold; background: url('/images/citembg.png') no-repeat 0 0;</xsl:when>
                     <!-- Иначе обычный вывод с пустым стилем -->
                     <xsl:otherwise></xsl:otherwise>
                  </xsl:choose>
               </xsl:variable>
               
               <a onClick="return show_hide_menu('{@id}');" href="{/shop/path}{fullpath}" style="{$link_style}">
                  
                  <xsl:choose>
                     <xsl:when test="propertys/property/node()">
                        <xsl:value-of disable-output-escaping="yes" select="propertys/property/value"/></xsl:when>
                     <xsl:otherwise>
                        <xsl:value-of disable-output-escaping="yes" select="name"/>
                     </xsl:otherwise>
                  </xsl:choose>
               </a>
               <!--
               <xsl:if test="count_all_groups > 0">
                  <ul class="left_menu_2">
                     <xsl:apply-templates select="group"/>
                  </ul>
               </xsl:if>
               -->
            </xsl:when>
            <xsl:otherwise>
               <a onClick="return show_hide_menu('{@id}');" href="{/shop/path}{fullpath}">
                  <xsl:choose>
                     <xsl:when test="propertys/property/node()">
                        <xsl:value-of disable-output-escaping="yes" select="propertys/property/value"/></xsl:when>
                     <xsl:otherwise>
                        <xsl:value-of disable-output-escaping="yes" select="name"/>
                     </xsl:otherwise>
                  </xsl:choose>
               </a>
            </xsl:otherwise>
         </xsl:choose>
         <!-- Если есть подгруппы -->
         <xsl:if test="group">
            <ul id="{@id}" style="display: none;" class="left_menu_2">
               <xsl:apply-templates select="group"/>
            </ul>
            
         </xsl:if>
      </li>
   </xsl:template>
</xsl:stylesheet>

count(ancestor::имя узла)

<xsl:if test="count(ancestor::group) = 2">
...
</xsl:if>