Quote from the XPath Recommendation:
The + operator performs addition.
I think your example doesn't work because the xsl:value-of element can't have a "name" attribute.
Later: Oh, I see, you want to know how to assign the sum to a variable named "var3". You use the <xsl:variable> element to declare a variable and assign a value to it. Remember that these two must always be done at the same time in XSLT, you can't declare a variable and later assign it a value. So:
[ May 24, 2006: Message edited by: Paul Clapham ]