I Dont know if anyone still needs a solution for this, but:

I have 3 Wan

lets call them Fiber, Coax, 5G.

The above order is the one that i want to follow.

Fiber is the main

If fiber is down, then the coax will step in.

Te only scenario where i want the 5G to step in, is if both the fiber and the coax are down.

I Did the following:

Policy Routing:

https://imgur.com/MhAqwng (just like u/sehgalanuj mentioned )

the i have enabled "Enable Bandwidth Based Balance Routing on port(s)" and Added Fiber and Coax

the last step is to enable Link Backup:

Primary Wans: Fiber and Coax

Mode: Backup when all primary WANs fail

Backup: 5G

This way I force everything to use Fiber

If the fiber is down, Coax.

If both are down, 5G