gistfile1.xml

<xsl:template match="data">
	<xsl:variable name="number-of-entries" select="count(my-datasource/entry)"/>
	<xsl:variable name="per-column" select="ceiling($number-of-entries div 3)"/>

	<xsl:for-each select="my-datasource/entry[position() mod ($per-column - 1)]">
		<xsl:variable name="position" select="position()"/>

		<xsl:variable name="start" select="($per-column * $position) - ($per-column - 1)"/>
		<xsl:variable name="end" select="($per-column * $position)"/>
	
		<xsl:variable name="entries" select="/data/my-datasource/entry[position() &gt;= $start and position() &lt;= $end]"/>
	
		<xsl:if test="$entries">
			<ul>
				<xsl:apply-templates select="$entries" mode="list-item"/>
			</ul>
		</xsl:if>

	</xsl:for-each>
</xsl:template>

<xsl:template match="entry" mode="list-item">
	<li>...</li>
</xsl:template>