http://packetpros.com/wiki/index.php/How_do_I_configure_SNMPv3_on_a_router%3F (may have to cut-n-past URL)
Or the quick and dirty-
snmp-server view ORIONView system included
snmp-server view ORIONView internet included
snmp-server view ORIONView interfaces included
snmp-server view ORIONView chassis included
snmp-server group AcmeGroup v3 auth read ORIONView
snmp-server user someuser AcmeGroup v3 auth md5 SeCrEt priv aes SeCrEt2
Hope it helps.