Using hostname based SSL sites has many limitations. Thomas Deml wrote a blog http://blogs.iis.net/thomad/archive/2008/01/25/ssl-certificates-on-sites-with-host-headers.aspx that explains some of the issues. Basically when client connects to server, the server only knows what server's IP address client connected to. So it cannot choose the server certificate based on hostnames, but only based on IP address. In controlled environment where multiple sites are closely related and it is OK for them to share certificate (assuming that wildcard certificate is used or it contains multiple names).
One of your 3 sites (assuming they are related should own the binding without the hostname and that would allow the association of the certificate with the endpoint (this certificate would then be used for all the other sites that share the port and differ only in the endpoint.
Note: This shared ownership of SSL certificate is rather confusing, but the only way around it would be for server to know about the hostname chosen by client before certificate is chosen by server. There is a "TLS name extension" (RFC 3546) spec that supports that but it will still take few years before clients and servers adopt it so that is is actually useful.