I'm writing a song with the chords Bbmaj, Gmin, and C7, respectively. I want to write more melodies and progressions to go along with this but I am confused as to what scale it corresponds with. G Harmonic Minor? But it sounds like it resolves on D7 so I was wondering if it was a mode of D.
Don't worry about the scale. All you need is the key and chords. Play those.
Technically you can play a lot of different scales over that same progression. You can play even more scales over each individual chord. I'd suggest what cdgraves said and just worry about key and chords. Even in that, you can still modulate to a different key if you use the right transition chords.
you can play alot f things over alot ofthings

but if it helps those chords are all in the key of F it sounds like it resolves to a D becasue the C7 to a D is a deveptive cadence
so try using D minor(harmonic and melodic)
of F major over that

i dont have a guitar with me to play it but just some suggestions