PayPal Payments Pro is not the only option that is available for PayPal checkout in PowerStore 3. In the settings you can also choose PayPal Express Only. This is the equivalent of Payments Standard as the user will have to login to PayPal before confirming the order. This is a much better choice than the regular Payments Standard, I would not suggest making use of Payments Standard if it is not needed.
If you use this checkout option you can choose to test with the sandbox or putting the store in test mode. If you are getting to the pp_checkout_failure page then you will need to add those debug bindings to the page and if you get the message about the xml right click and choose view source. If you have any troubles get us this information minus any sensitive details and we will look into the problem with you.