When an AutoDiscovery Configuration is run, Spectrum tests the community strings starting with the first community string at the top of the list and works it's way down the list testing until it gets either a response from one of the community strings in the list or all in the list fail. What is happening is Spectrum gets an authentication failure response from the device when using the V3 community string. Spectrum is misinterpreting the authentication failure response from the device as a valid response and does not attempt the other community strings in the list.
If the AutoDiscovery debug is enabled during this discovery, you will see messages similar to the following in the ADISC debug file where Spectrum is attempting to model the device using the V3 community string even though the device at that ip address is not configured with that v3 community string
11:15:12 IP Discovery 22.214.171.124: mtype identification failed at community name #v3/P:AuthPassword:PrivPassword/USER and SNMP port 161 with error: 1