Irule with elseif and pool selection

The problem you have is you are starting with a switch and confusing the runtime with if statements.

If you want to use If's, then it would look more like this:

when HTTP_REQUEST { 
    set hostname [string tolower [HTTP::host]]
    if { $hostname starts_with "abc" } { 
        pool ss-devsp3-pool
    } elseif { $hostname starts_with "def" } { 
        pool ss-devsp3-pool-81
    } elseif { $hostname starts_with "ghi" } { 
        pool ss-devsp3-pool-82
    }
}

Using the switch statement, it would look more like this:

when HTTP_REQUEST { 
    switch -glob [string tolower [HTTP::host]] {
        abc*    {  pool ss-devsp3-pool }
        def*    {  pool ss-devsp3-pool-81 }
        ghi*    {  pool ss-devsp3-pool-82 }
    }
}