It sounds like KaerfSusej has found the chord he/she wants, but I noticed something that suprised me.
Kirk brought up that Am is the 1 chord of the key of A Minor (or A Aeolian), and the C major and G majnor chords are also in that key. Major and Minor (or Ionian and Aeolian respectively) are just different starting points of one patter and Major and Minor are no more elementary/basic/important than the other starting points (except for Ionian and Aeolian are the most common). Like Kirk said Em and F and Dm are the likely contenders. I noticed that Am is the 1 chord of A Dorian and the C and G chords are in A Dorian, Em is also in A dorian. Now the thing that pleasently suprised me: the D Major chord is the 5 chord of A Dorian which was also suggested. (also A Dorian will add Bm as a likely contender but then F and D wouldnt be relative chords with Am and C and G).
I like it how everything connects together

If you learn how to play songs, then you learn songs. If you learn how to improvise, then you learn music.