Firstly, there is such a thing called 'Funtional Harmony', which is your I, IV, V chord. I- means the 1st degree chord; IV- means the 4th degree chord; and V- means the fifth degree chord. If it were applied in the key of C major, the chords are C major (I), F major (IV), and G major (V).
In the same way, if we look at its relative minor (which is the 6th degree of the major scale relative to the root), then we get a functional harmony of vi, ii, iii. Therefore we have A minor, D minor, and E minor. Its the same as i, iv, v if we start with the note A (A,B,C,D,E,F,G), which still gives you A minor, D minor, and E minor in a I, IV, V progression.
Therefore, all the chords in the functional harmony have the same tonality (major or minor) in its basic triad construction.
Lastly, to have a better grasp of the idea of progressions and fuctional harmony, you need to know about the two approaches when studying modes. These are the Relative (or Derivative) Approach and the Parallell Approach.
For simplicity, lets just use the key of C for examples.
In the Relative Approach, modes are derived at different starting points of the scale. In other words, the different modes of scale have the same notes. The Parallell Approach, the other hand, has the same root note but they are of different scales. Let me illustrate:
Relative Approach (Key of C):
Ionian :C - D - E - F - G - A - B
Dorian :D - E - F - G - A - B - C
Phrygian :E - F - G - A - B - C - D
Lydian :F - G - A - B - C - D - E
Mixolydian :G - A - B - C - D - E - F
Aeolian :A - B - C - D - E - F - G
Locrian :B - C - D - E - F - G - A
Parallell Approach:
Ionian :C - D - E - F - G - A - B
Dorian :C - D - Eb - F - G - A -Bb
Phrygian :C - Db - Eb - F - G - Ab -Bb
Lydian :C - D - E - F# - G - A -B
Mixolydian :C - D - E - F - G - A -Bb
Aeolian :C - D - Eb - F - G - Ab -Bb
Locrian :C - Db - Eb - F - Gb - Ab -Bb
Well, I hope this has been beneficial.
