we have 3 external web urls which will come on our frontend f5 and redirect to internal web urls and use as a revers proxy

It's interesting how often this comes up, and the most important thing to understand here is the (potentially incorrect) use of the term "redirect". In HTTP-speak, and equally in BIG-IP iRules, a redirect is a flow control command. Given a typical reverse proxy configuration, where a client request flows through the proxy to the server, a redirect inserted at the proxy causes a preempt response back to client without passing traffic to the server. I'm assuming that's not what you want.

So based on your description, there are at least three requirements:

1. Terminate the client side SSL and pass server side HTTP unencrypted
2. Parse HTTP requests
3. Modify the HTTP Host request header

The first is accomplished by incorporating a client SSL profile. The second is accomplished via attached HTTP profile. And the third can be a fairly straight forward iRule. I'm assuming here that all three external URLs are coming to the same virtual server and iRule.

when HTTP_REQUEST {
    switch [string to lower [HTTP::host]] {
        "site1ex.com" {
            HTTP::host "site1in.com"
        }
        "site2ex.com" {
            HTTP::host "site2in.com"
        }
        "site3ex.com" {
            HTTP::host "site3in.com"
        }
        default {
            HTT::host "site1in.com"
        }
    }
}