Required help for irule on single url

I'd probably leave it the same except for the pool definitions

pl_abc_http members {2.2.2.2:80}
pl_def_http members {2.2.2.3:80}
pl_ghi_http members {2.2.2.4:80}

Slight tweak to iRule to show you some other things you might want to do;-

when HTTP_REQUEST {
    set path_seg1 [lindex [split [HTTP::path] / ] 1 ]
    switch $path_seg1 {
        "abc" -
        "def" -
        "ghi" {
             Rewrite Host header
            HTTP::header replace Host "${path_seg1}.com"
             Rewrite path to remove first segment (not sure if this is actually your requirement)
            if {[substr [HTTP::path] 4 ] starts_with "/"} {
                HTTP::path [substr [HTTP::path] 4 ]
            } else {
                HTTP::path "/"
            }
            pool "pl_{$path_seg1}_http"
        }
        default {
             Return 404 for not found path
            HTTP::respond 404 noserver content {404 - Page Not FoundWe're sorry - the page you are looking for cannot be found}
            return 
        }
    }
}