3 tier web app setup

I have been tasked to build a 3 tier web application environment. They want the 3 servers to be Windows using IIS 8+ and mysql as the DB. The application has been written in php. I’m not sure where to start as far as splitting the web/php/application apart. Which server (web or application) should php be installed on? I have heard conflicting opinions. What method should be used to allow the web server access to the application on the app server?

I would appreciate any assistance in pointing me in the right direction to set this up.