What scale to use? All three chords occur in the key of G major, so use your scale on the 12th fret (or on the ‘0’ fret), because when you play it there, it is the G Major scale.
8
C
C
5
1
Am
-OR-
Am
F
F
3
G
G
What scale to use? The C Major scale since all four chords are in the key of C.
7
Em
-OR-
Em
5
A
A
What scale to use? Both Em and A are in the key of D, so use the D Major scale.