Before you start, make sure that you have SNMP V3 already setup on your device.  You can refer to this post if you need to set it up.

Add the device to Zenoss, and then edit the devices Configuration Properties, you’ll need to configure the following settings.

Zenoss setting name        Options
zSnmpSecurityName          [user]
zSnmpPrivType                   (AES or DES)
zSnmpPrivPassword          PASSPHRASE
zSnmpAuthType                  (MD5 or SHA)
zSnmpAuthPassword         PASSPHRASE
zSnmpVer                           v3

After all the above changes are made, Zenoss should start graphing the device.

The chart below shows which snmpwalk options match which Zenoss Configuration Properties.

snmpwalk     Zenoss Configuration     snmpd.conf
-v                   zSnmpVers                        n/a
-u                   zSnmpSecurityName        createUser [user]
-x                   zSnmpPrivType                 (AES or DES)
-X                  zSnmpPrivPassword         PASSPHRASE
-a                  zSnmpAuthType                 (MD5 or SHA)
-A                  zSnmpAuthPassword        PASSPHRASE

You can refer to the Zenoss wiki for more info.