How to create iRule to grant additional access to a specific client

I'd suggest you change the logic a little and use a data group to keep the code simple. I'd make bat1_test2 the VS default pool and use this iRule after creating a DG call 'allowed_ips' with those three addresses in it;

when CLIENT_ACCEPTED { 
 if { [class match [IP::client_addr] equals allowed_ips] } {
  pool bat1_test1 }
}