The 3 files required to be modified are:
what sort of files are the 3 that you need to modify and how will that impact the installation of other contributions?
Modified to show the Express Checkout Button in the first step of checkout process (required by PayPal).
Modified to handle shipping address error sent back from PayPal.
Modified to handle shipping address changed at PayPal, and update the shipping cost.
Modified to handle 'virtual' orders (download items).
Modified to handle payments cancelled at PayPal.
Modified to handle transaction error message from PayPal.
Added 2 lines of filename definitions.
The modifications are all well commented, and it's very easy to identify them when applying other contributions. If you need help for applying other modifications in checkout_shipping.php, you can post in this thread.