Ian,
I have configured SNMP v3 without any issues.
if possible, please provide a "show snmp summary" from the APIC CLI.
Additional information here:
Technote: SNMP in the ACI Fabric
https://supportforums.cisco.com/document/12961146/technote-snmp-aci-fabric
Ask the ACI Experts: SNMP in the ACI Fabric
https://supportforums.cisco.com/blog/13100731/ask-aci-experts-snmp-aci-fabric
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Thank you!
T.