how to do http url monitoring for three servers having more than one website?

If I understand you correctly, you're hosting multiple applications on the same server nodes under different URLs. If that's correct, probably one of the easiest things to do would be to create separate pools for each application (with the same pool members) and apply the most appropriate health monitor to each. Example:

pool1
    10.10.10.1
    10.10.10.2
    10.10.10.3
        monitor: HTTP GET /app1

pool2
    10.10.10.1
    10.10.10.2
    10.10.10.3
        monitor: HTTP GET /app2

This has the added benefit of letting you scale the individual applications separately. Then you'd just use an iRule, HTTP class (< 11.4), or policy (>=11.4) to switch between the pools based on URL:

when HTTP_REQUEST {
    switch [string tolower [HTTP::host]] {
        "app1.domain.com" { pool pool1 }
        "app2.domain.com" { pool pool2 }
    }
}