IRule Creation help

What do you want the user persisted to? Do you want to ensure that once they go to poolA they always go to pool A? If so you could use your own cookie to do this:

when HTTP_REQUEST {
    Check if there is a pool selector cookie in the request
    Use a switch statement to ensure only valid pool names are present
    instead of accepting any pool name from the user supplied cookie
   switch [HTTP::cookie pool_cookie] {
      poolA -
      poolB -
      poolC {
          Select the pool from the cookie
         pool [HTTP::cookie pool_cookie]
         set selected ""
      }
      default {
          No pool selector cookie, so randomaly choose a pool
          Save a random number between 0 and 1
         set rand [expr { rand() }]
         if { $rand < .90 } { 
            pool poolA
            set selected poolA
         } elseif { $rand < .95 }{
            pool poolB
            set selected poolB
         } else {
            pool poolC
            set selected poolC
         }
      }
   }
}
when HTTP_RESPONSE {
    Set a pool selector cookie if a pool was selected for this request
   if {$selected ne ""}{
      HTTP::cookie insert name pool_cookie value $selected path "/"
   }
}

If you need persistence within the pool members you could use a default cookie insert persistence profile. This will "just work" even when using multiple pools on the same virtual server.

Aaron