A basic thing is spell the chords (correctly) then put the letters all in order...
G = G B D
A = A C# E
Bm = B D F#
A B C# D E F# G A
...then look for a key/scale. In this case you have
C#. If you know your key signatures, you will recognize that's
D major or
Keep in mind the importance of knowing key signatures. You are working by trial and error, probably because you haven't learned key signatures. If you really want to identify scales and keys, you must learn them.
Another approach is to use basic harmonic patterns. You need to know important diatonic chord patterns to do it - the circle of fifths progression and either
II V I or
IV V I cadences in major and minor keys is the essential stuff to know.
Two major chords with roots separated by a whole step occur in only one place within diatonic harmony. It also happens the chord on the next root up a whole tone will be minor (in diatonic harmony.) Those are clue to start zeroing in on the scale/key/tonic. We know the three chords fit a basic diatonic pattern.
Ultimately you want to determine the tonic, the central tone of a key, starting tone of a scale. Sometimes that can be tricky. Music is sometimes ambiguous about key, mode, etc. A lot will depend on the style conventions of the music. I think the style you are working with is probably rock, and that helps us.
♭VI ♭VII i is a very common pattern in rock music. By way of contrast something like
ii6 V I is a common harmonic formula in classical music where the
V chord is critical for determining a key. Rock music very often eschews the
V I progression and does other things ...like
♭VI ♭VII i.
If we consider
B the tonic, then we could have...
Bm: ♭VI ♭VII i
It's very helpful that you gave duration for the chords, you specifically have...
Bm: |♭VI |♭VI |♭VII|♭VII|i |i |i |i |
...the longer duration of
Bm lends weight to calling it the
i chord, the tonic chord.
So, the "scale" is
Notice I put "scale" in quotes. It's probably better to think in terms of
B is the tonic, or the tonality is
B minor, and a whole variety of scales and embellishments of the chord tones could be used.
I tried the B blues scale? And it sounded really good. Is that a major or minor pentatonic scale? Speaking of outside Notes . Notes in B Phrygian dominant scale did not seem to fit quite well.
B C D# E F# G A B
This is where the key signature knowledge - and learning the gamut of letters by thirds and perfect fifths - will work for you. Tetrachord knowledge helps with this particular one.
Phrygian mode is characterized by a lowered second scale degree, a half step above the tonic. The major and minor scales don't do that, they have the second degree a whole step above the tonic. So phrygian is basically ruled out from the start.
Also, phrygian dominant starts with a harmonic tetrachord and that contains a major third above the tonic. This makes phrygian dominant a bit of an odd bird. It sounds minor-ish, because it's a mode of the harmonic minor scale, but it's tonic chord is major. The chord progression uses
Bm. Phrygian dominant's tonic
B major chord contradicts the tonic of the chord progression.
Specifically there are two points of clashing: the
C of the scale and
C# of the
A chord and the
D of the
Bm chord against the
D# of the scale.