Loadbalance redirects

Well, my first recommendation, if it's possible, would be to create a pool with the three systems in it, and just let BIG-IP load balance normally to that pool. You wouldn't need an iRule at all for this.

If, for some reason, you can't use a pool, and need the explicit redirect in the iRule, then you'd just need something like a simple counter to spread out the requests in a round robin fashion, like this:

*edited to include unruley's suggestion below*

when RULE_INIT {
  set ::n 1
}
when HTTP_REQUEST {
  if { $::n >= 4 } {
    set n 1
  }
  HTTP::redirect "https://www$n.server.com"
  incr ::n
}

-Colin