Originally Posted by: HambergIn any given key the 3rd interval is a modal scale step. So that means it can be flattened to a minor interval without changing the key. But wouldn't flattening the 3rd make the entire key minor. Because with a major third the 1st triad has the root third and fifth, while with a minor third the 1st triad has the root minor third and fifth?

This doesnt make sense to me.

The third degree of the scale, is not the only one that is used to play modes; you can play modes starting on any degree of the scale.
Another way to look at modes, is thinking of them as inversions of the Scales.
[C-D-E-F-G-A-B]= Natural Major Scale or Ionian Mode.
[D-E-F-G-A-B-C]= Dorian Mode.
By making the inversion shown above, you would be playing a " D Dorian",
If you change the tonic to the third degree {E} you get the Phrygian Mode.

Now, there is nothing wrong in playing a "D Dorian" or an "E Phrygian" over a
"C Major Chord".
But you get the best of the modal sound, when you play a chord built on the root of the mode.
Example:
D Dorian=.......[D-E-F-G-A-B-C]
D Minor Chord=[D-F-A]
Dm7 Chord=....[D-F-A-C].
To get the flavor of the Dorian Mode, play the mentioned mode over a "D minor Chord" or better yet; a "Dm7 Chord"

But wouldn't flattening the 3rd make the entire key minor.

In order to change the major scale to a minor scale, you need to flat the third degree, the sixth degree and the seventh degree.