how to create three tier web application using web services

I hope you know about the 3 tiers:

1. Presentation

2. Business rules

3. Data Access

As per your question, you should move your entire business rules tier to your Web service. So the rest of the tiers will be unchanged. However, you should configure a little between these layers. Since you are introduced a web service. Not much changes don't worry.

This is how enterprise application works. The main reason, why we move Business rules to Web service or WCF is, this layer can be access by any other application in other technologies like java or any other web technologies.

Hope this helps!