I have a 3 tier farm with Central Admin on the wfe (HA - everthing x2). I'm looking at the best way to install SSRS. What parts need to be installed where. From what I have worked out Reporting Services needs to be installed on the app server and RS add in needs to be installed on the WFE. Is this correct? Also where is best to configure the report service application where it should be on the app or wfe
There's a well documented set of steps for installing Reporting Services on SharePoint 2013 at Install Reporting Services SharePoint Mode for SharePoint 2013
Note that you should follow the links near the top of the page that discuss adding an additional Report Server to the farm (SSRS scale out) and adding an additional reporting services web front-end to a farm as these discuss in detail where the components/add-in should be installed and where to run services.
Yes, you should install the Reporting Services in SharePoint mode on the application servers and the Reporting Services add-in on the web front end servers in the farm (I double-checked some notes from some time ago from an installation that I did and found that I'd ended up installing both the Reporting Services components and the add-in on all servers in the farm and vaguely remember that I had to do this as the health analyser rules complained if I didn't have all components on everything for some reason, but YMMV). I'd follow the guidance at the URLs above in the first instance, and keep an eye on the health analyser output to see whether anything is flagged following installation and configuration.
Counter to Andy's answer, SSRS will provide better end user performance by being deployed to the FEs the users are interfacing with. This may require additional resources to be allocated to the FEs, but with equal resources available (CPU, RAM), it will perform better on the FE.