Grouping the nodes at three levels and summing one

Please follow the comments in the sample xml it explains everything I need, it is having three scenerio's check each scenerio in the output xml in the same sequence.

Input XML

   <!-- In the below xml there will be one ASNInDesc with multiple ASNInPO's 
        and each ASNInPO contains one ASNInCtn and each ASNInCtn contains one ASNInItem -->

<ASNInDesc>
   <asn_nbr>ASN-1</asn_nbr>

   <!-- In the below two ASNInPO's po_nbr is same container_id under ASNInCtn is same and item_id under
        ASNInItem is same. In this case two ASNInPO's has to be merged and two ASNInCtn's has to be merged
        into one tag(since the container_id is same) and two ASNInItem's has to be merged into one tag and unit_qty is to be added  -->

   <ASNInPO>
      <po_nbr>PO-1</po_nbr>
      <ASNInCtn>
         <container_id>CONTAINER-1</container_id>
         <ASNInItem>
            <item_id>ITEM-1</item_id>
            <unit_qty>2</unit_qty>
         </ASNInItem>
      </ASNInCtn>
   </ASNInPO>
   <ASNInPO>
      <po_nbr>PO-1</po_nbr>
      <ASNInCtn>
         <container_id>CONTAINER-1</container_id>
         <ASNInItem>
            <item_id>ITEM-1</item_id>
            <unit_qty>2</unit_qty>
         </ASNInItem>
      </ASNInCtn>
   </ASNInPO>

   <!-- In the below two ASNInPO's po_nbr is same container_id under ASNInCtn is same and item_id under
        ASNInItem is different. In this case two ASNInPO's has to be merged and two ASNInCtn's has to be merged into one tag and
        two different ASNInItem's for the two different items  -->

    <ASNInPO>
      <po_nbr>PO-2</po_nbr>
      <ASNInCtn>
         <container_id>CONTAINER-2</container_id>
         <ASNInItem>
            <item_id>ITEM-2</item_id>
            <unit_qty>3</unit_qty>
         </ASNInItem>
      </ASNInCtn>
   </ASNInPO>
   <ASNInPO>
      <po_nbr>PO-2</po_nbr>
      <ASNInCtn>
         <container_id>CONTAINER-2</container_id>
         <ASNInItem>
            <item_id>ITEM-3</item_id>
            <unit_qty>3</unit_qty>
         </ASNInItem>
      </ASNInCtn>
   </ASNInPO>

   <!-- In the below two ASNInPO's po_nbr is same container_id under ASNInCtn is different and item_id under
        ASNInItem is different. In this case two ASNInPO's has to be merged into one tag with
        two different ASNInCtn's each having their own ASNInItem  -->

   <ASNInPO>
      <po_nbr>PO-3</po_nbr>
      <ASNInCtn>
         <container_id>CONTAINER-3</container_id>
         <ASNInItem>
            <item_id>ITEM-3</item_id>
            <unit_qty>2</unit_qty>
         </ASNInItem>
      </ASNInCtn>
   </ASNInPO>
   <ASNInPO>
      <po_nbr>PO-3</po_nbr>
      <ASNInCtn>
         <container_id>CONTAINER-4</container_id>
         <ASNInItem>
            <item_id>ITEM-3</item_id>
            <unit_qty>2</unit_qty>
         </ASNInItem>
      </ASNInCtn>
   </ASNInPO>

</ASNInDesc>

Output XML

<?xml version = '1.0' encoding = 'UTF-8'?>
<ASNInDesc>
   <asn_nbr>ASN-1</asn_nbr>
  <!-- Scenerio-1 --> 
   <ASNInPO>
      <po_nbr>PO-1</po_nbr>
      <ASNInCtn>
         <container_id>CONTAINER-1</container_id>
         <ASNInItem>
            <item_id>ITEM-1</item_id>
            <unit_qty>4</unit_qty>
         </ASNInItem>        
      </ASNInCtn>
  </ASNInPO>
  <!-- Scenerio-2 -->  
  <ASNInPO>
      <po_nbr>PO-2</po_nbr>
      <ASNInCtn>
         <container_id>CONTAINER-2</container_id>
         <ASNInItem>
            <item_id>ITEM-2</item_id>
            <unit_qty>3</unit_qty>
         </ASNInItem>
         <ASNInItem>
            <item_id>ITEM-3</item_id>
            <unit_qty>3</unit_qty>
         </ASNInItem>        
      </ASNInCtn>
   </ASNInPO>
  <!-- Scenerio-3 -->   
   <ASNInPO>
      <po_nbr>PO-3</po_nbr>
      <ASNInCtn>
         <container_id>CONTAINER-3</container_id>
         <ASNInItem>
            <item_id>ITEM-3</item_id>
            <unit_qty>2</unit_qty>
         </ASNInItem>
      </ASNInCtn>
      <ASNInCtn>
         <container_id>CONTAINER-4</container_id>
         <ASNInItem>
            <item_id>ITEM-3</item_id>
            <unit_qty>2</unit_qty>
         </ASNInItem>
      </ASNInCtn>     
   </ASNInPO>

I have tried with my own xsl, but couldn't handle all the scenerios. Please help me in solving this. Thanks in Advance.

Here's a XSLT 1.0 stylesheet using Muenchian grouping. This is based on my answer to your other question.

This is the same approach, but with adjusted keys where we use a concatenation of the element's own id and the parent elements' id to look up elements for grouping. For example, ASNInItem elements are looked up using the following concatenated key: concat(../../po_nbr, '|', ../container_id, '|', item_id). The choice of separator is not important, but you need to choose a character which does not appear in the ids.

Also the summing of unit quantities has been added.

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet version="1.0" 
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

  <xsl:output method="xml" indent="yes" encoding="UTF-8"/>

  <xsl:key name="ASNInPO" match="ASNInPO" 
           use="po_nbr"/>

  <xsl:key name="ASNInCtn" match="ASNInCtn" 
           use="concat(../po_nbr, '|', 
                       container_id)"/>

  <xsl:key name="ASNInItem" match="ASNInItem" 
           use="concat(../../po_nbr, '|', 
                       ../container_id, '|', 
                       item_id)"/>

  <xsl:template match="ASNInDesc">
    <xsl:copy>
      <xsl:copy-of select="asn_nbr"/>
      <xsl:apply-templates 
          select="ASNInPO[generate-id() =
                          generate-id(key('ASNInPO', 
                                          po_nbr)[1])]"/>
    </xsl:copy>
  </xsl:template>

  <xsl:template match="ASNInPO">
    <xsl:copy>
      <xsl:copy-of select="po_nbr"/>
      <xsl:apply-templates 
          select="key('ASNInPO', po_nbr)/
                  ASNInCtn[generate-id() =
                           generate-id(key('ASNInCtn',
                                           concat(../po_nbr, '|', 
                                                  container_id))[1])]"/>
    </xsl:copy>
  </xsl:template>

  <xsl:template match="ASNInCtn">
    <xsl:copy>
      <xsl:copy-of select="container_id"/>
      <xsl:apply-templates
          select="key('ASNInCtn',
                      concat(../po_nbr, '|', 
                             container_id))/
                  ASNInItem[generate-id() =
                            generate-id(key('ASNInItem', 
                                        concat(../../po_nbr, '|',
                                               ../container_id, '|',
                                               item_id))[1])]"/>
    </xsl:copy>
  </xsl:template>


  <xsl:template match="ASNInItem">
    <xsl:copy>
      <xsl:copy-of select="item_id"/>
      <unit_qty>
        <xsl:value-of 
            select="sum(key('ASNInItem', 
                        concat(../../po_nbr, '|', 
                               ../container_id, '|', 
                               item_id))/unit_qty)"/>
      </unit_qty>
    </xsl:copy>
  </xsl:template>

</xsl:stylesheet>