Lesley,
First thing is to check the snmp v3 configuration on device and query it from Mib tools
Configuring SNMP on Devices Running the Junos OS - Technical Documentation - Support - Juniper Networks
Things to remember:
SPECTRUM supports the following:
Authentication - SHA, MD5 (MD5 by default)
Privacy - AES, 3DES, DES (DES by default)
You can override a protocol by specifying the protocol in the auth or priv string as follows:
SHA^<authstring> will use SHA for the auth string instead of the default MD5
AES^<privstring> will use AES for the priv string instead of the default DES
You can also change the default protocols by overriding them in the SPECTRUM .vnmrc file with the following lines:
snmpv3_default_auth_protocol=***
snmpv3_default_priv_protocol=***
HTH
Kalyan