I'm afraid you are right - Microsoft does not support SNMP v3 in their Windows SNMP service. There are replacement services available, 3 I have come across:
Mg-Soft which starts @ $98 for a single computer, $300 for 10.
NuDesign $500 for 25.
Marksw $20/each (looks like a single developer)
I have not tested any of these, so cannot comment on their quality!