Multiple Pools on Single Virtual Server

Your "Original CName URL" statement makes me think you're talking about separate DNS-based host names, in which case a variation of giltjr's iRule might be fitting:

when HTTP_REQUEST {
    switch [string tolower [HTTP::host]] {
        "sitea.domain.com" { pool sitea_pool }
        "siteb.domain.com" { pool siteb_pool }
        "sitec.domain.com" { pool sitec_pool }
    }
}