<?xml version="1.0"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
 <xsl:output method="text"/>

 <xsl:template match="/">
  <xsl:for-each select="abeceda/*">
   <xsl:sort data-type="text" select="@c"/>
   <xsl:value-of select="."/>
   <xsl:value-of select="@c"/>,
  </xsl:for-each>
   ///
  <xsl:for-each select="abeceda/*">
   <xsl:sort data-type="number" select="@c"/>
   <xsl:value-of select="."/>
   <xsl:value-of select="@c"/>,
  </xsl:for-each>
   ///
  <xsl:for-each select="abeceda/*">
   <xsl:sort order="descending" select="."/>
   <xsl:value-of select="."/>,
  </xsl:for-each>
 </xsl:template>

</xsl:stylesheet>