There are several fallback scenarios which can be used.
In the HA scenario there is a fallback for every load balancer.
In this scenario the load balancers are fallback for each other.
2 Active, 1 Fallback
In this scenario you have two active load balancers and one fallback for both.
In every case the network numbers are more important than the order.
In this scenario Y takes over if X fails, because network A is not assigned to another instance. Z works as passive standby. If X and Y fail, Z takes over.
In this scenario Z is fallback for X and Y because both networks are also assigned to Z.
|Instance Z||3||A, B|