Here's the a step by step guide for configuring a basic working SNMPv3 template via Mist:

A) Configure General

B) Configure USM:

C) Configure VACM

• Access and Security Group:

D) Configure Notify and Notify Filter (Optional)

E) Configure Target Parameters

F) Configure a View:

Above parameters pushed via Mist, look like this in configuration set commands:

set groups top snmp v3 usm local-engine user prtg authentication-sha authentication-key "xxxxx"
set groups top snmp v3 usm local-engine user prtg privacy-des privacy-key "yyyyyy"
set groups top snmp v3 vacm security-to-group security-model usm security-name prtg group PRTG
set groups top snmp v3 vacm access group PRTG default-context-prefix security-model usm security-level authentication read-view PRTG
set groups top snmp v3 vacm access group PRTG default-context-prefix security-model usm security-level authentication write-view PRTG
set groups top snmp v3 vacm access group PRTG default-context-prefix security-model usm security-level authentication notify-view PRTG
set groups top snmp v3 target-address PRTG-Probe address 10.23.2.25
set groups top snmp v3 target-address PRTG-Probe port 161
set groups top snmp v3 target-address PRTG-Probe address-mask 255.255.255.0
set groups top snmp v3 target-address PRTG-Probe target-parameters PRTG_V3
set groups top snmp v3 target-parameters PRTG_V3 parameters message-processing-model v3
set groups top snmp v3 target-parameters PRTG_V3 parameters security-model usm
set groups top snmp v3 target-parameters PRTG_V3 parameters security-level authentication
set groups top snmp v3 target-parameters PRTG_V3 parameters security-name prtg
set groups top snmp v3 target-parameters PRTG_V3 notify-filter Juniper
set groups top snmp v3 notify PRTG type trap
set groups top snmp v3 notify PRTG tag PRTG
set groups top snmp v3 notify-filter Juniper oid .1 include
set groups top snmp engine-id local mist
set groups top snmp view PRTG oid .1 include

Here a brief explanation for each of the commands:

1. User Configuration:

set snmp v3 usm local-engine user prtg authentication-sha authentication-key "xxx"

          Creates a user called prtg with SHA authentication. The authentication-key is the password used for this authentication.

set snmp v3 usm local-engine user prtg privacy-des privacy-key "xxx"

          Adds DES privacy (encryption) for the user prtg. Privacy type must match the server's. The privacy-key is the password used for encryption. 

2. Access Control:

set snmp v3 vacm security-to-group security-model usm security-name prtg group PRTG

          Maps the user prtg to the PRTG group using the USM security model.

set snmp v3 vacm access group PRTG default-context-prefix security-model usm security-level authentication read-view PRTG

          Grants read access to the PRTG group. This means users in this group can read SNMP data. 

set snmp v3 vacm access group PRTG default-context-prefix security-model usm security-level authentication write-view PRTG

          Grants write access to the PRTG group. This allows users in this group to modify SNMP data. 

set snmp v3 vacm access group PRTG default-context-prefix security-model usm security-level authentication notify-view PRTG

          Grants notify (trap) access to the PRTG group. This allows users in this group to receive SNMP notifications. 

3. Target Address and Parameters:

set snmp v3 target-address PRTG-Probe address 10.23.2.25

          Sets the target address for SNMP notifications (Server's IP).

set snmp v3 target-address PRTG-Probe port 161

          (Optional) Specifies the port for SNMP notifications. If not specified it uses 161/162 by default.

set snmp v3 target-address PRTG-Probe address-mask 255.255.255.0

          Sets the address mask for the target, defining the range of addresses.

set snmp v3 target-address PRTG-Probe target-parameters PRTG_V3

          Associates the target address with the parameters PRTG_V3.

set snmp v3 target-parameters PRTG_V3 parameters message-processing-model v3

          Uses SNMPv3 message processing model.

set snmp v3 target-parameters PRTG_V3 parameters security-model usm

          Specifies USM as the security model.

set snmp v3 target-parameters PRTG_V3 parameters security-level authentication

          Sets the security level to authentication.

set snmp v3 target-parameters PRTG_V3 parameters security-name prtg

          Uses the security name prtg.

4. Notification Configuration:

set snmp v3 notify PRTG type trap

          Specifies the notification type as a trap.

set snmp v3 notify PRTG tag PRTG

          Tags the notification with PRTG for identification.

set snmp v3 notify-filter Juniper oid .1 include

          Includes all OIDs under .1 in the notification filter, meaning all relevant SNMP data will be included in notifications. 

5. Engine ID and View:

set snmp engine-id local mist

          Sets the local snmp engine ID to mist, uniquely identifying this SNMP engine on the device.

set snmp engine-id local mist

          Defines the view PRTG to include all OIDs under .1