I want to make sure I'm answering what you're asking. Are you asking about connecting a Roon Nucleus via USB directly to the BDA-3?
After doing some research, here's what's on Roon's website:
Connected Outputs: Built-in outputs, USB DACs, and Sound Cards
Any audio devices recognized by the OS will be recognized by Roon, from Built-in Sound to PCI Soundcards to USB DACs.
My guess is that you'll need to contact Roon to find out if the BDA-3 will be recognized by their OS. I have no idea what OS they are running or what drivers are available/supported. It's a good question though, as a BDP would be redundant.