Spell a major triad using pitch third triad?

In order to change a major (M) triad to a Minor (m) triad, simply lower the middle note a half step. For example, if you have the CM triad, you have Root (the bottom note): C 3rd (the middle note): E...

common chord

c b a g

I'm guessing you mean the chord/arpeggio? It's made up of the notes C, E, and G, the 1st, third and fifth notes of the scale :) Hope this helps (: