IRule for sending traffic to another server if the first server is down or second server is down

Priority group.

In the pool, set Priority Group Activation to "less than 1", then give each pool member a priority number. The higher the number the greater the priority. Example:

ServerA - priority 3
ServerB - priority 2
ServerC - priority 1

With Priority Group Activation, all traffic will be sent to priority number 3 servers until they all fail, then priority number 2 servers, and then priority number 1. Also makes sure you have a good monitor applied to the pool. No iRule required.