You have the quality of the chords wrong, it's actually a C maj7 - B min7 - Bb maj7.
It's disputable as to what scale you would use to identify this progression. Using a major 7th chord creates a feeling of amiguity in the listener, and it's not until later in the progression when you use more stable chords that you can get a feeling of a home key. Nevertheless, I'd argue that this is simply in the scale of C Major, making the roman numeral chord progression I maj7 - vi/ii - IV maj7/IV.