B flat
C
D
E flat
F
G
A
B flat
The subdominant in a scale is the 4th, which in the key of B-flat major is E-flat.
The mediant of B flat major is D.
B-flat
Eb Major
A-flat Major scale
Your question doesn't make much sense. But for any scale, "do" would be the first note of the scale, which is whatever the scale is called. But for a flat major, from what it sounds like you're asking, the note would be a flat. Ex: In B flat major, "Do" is b flat.
The mediant is the 3rd of the scale, so in B-flat it's D.
b flat, c, d, e flat, f, g, a, b flat
F Major
Eb Major
None. The F major scale has a B flat.
the scale e flat major has a flat, b flat and e flat.