LB behavior for backend IP connections

From the diagram you provided, it looks like you want to use 'layered virtual servers' (i.e. where traffic from one virtual server gets forwarded to another virtual server). You can do this based on the HTTP host header of the client request (assuming that SSL is terminated on the BIG-IP) using an iRule e.g:

when HTTP_REQUEST {
    set HOST [string tolower [HTTP::host]]

    switch $HOST {
        "<FQDN 1>"  { virtual <VS NAME> }
        "<FQDN 2>"  { virtual <VS NAME> }
        "<FQDN 3>"  { virtual <VS NAME> }

        default     { HTTP::respond 200 content "Invalid Hostname" }
    }
}