Hi @Vlad_Ghimboasa ,
For the MX appliance where you are attempting to utilize WAN3 as a Backup Uplink, are you able to confirm the firmware is on 18.2 or greater?
In order for the MultiWAN Backup Uplink feature to function as expected it does require the appliance be on firmware 18.2 or higher.
For more information regarding this feature, you can reference this doc as well: https://documentation.meraki.com/MX/MultiWAN_Backup_Uplink
If you found this post helpful, please give it kudos. If my answer solved your problem, click "accept as solution" so that others can benefit from it.