Click on > Inventory > Protocols and select the network you have set up with your smart bridge.
A new dialogue box should pop up that you can see in the screen shot below.
Select the SNMP protocol and set the Version to version 3. Additionally MDS authentication and DES encryption is set by default. You may have to modify these if your network is set up differently.
Once this is set could you attempt to back up your devices that are using SNMPv3.