Mysha, the 3rd in a triad is not "out of key," it is one of the notes of the scale.

For example:   C scale with tonic chord notes boldfaced

C    D    E    F    G    A    B    C

Within any triad, it is the 3rd which defines it as a major or minor chord (and it is not "too high" or "too low," it's exactly the pitch it is supposed to be). In the key of C, the C (tonic) chord is major, the D is minor, the E is minor, the F is major, the G is major, the A is minor, and the B is a diminished triad (root, minor third, and a diminished 5th rather than a perfect 5th). If a song is in major, the C, F, and G (or G7) are the primary chords, with the minor chords used as alternates or "color" chords. The G7 chord includes the diminished triad, along with it's own root note. If a song is in the relative minor (in this case, A minor), the three primary chords are Am, Dm, and Em (often replaced by an E7 to provide a leading tone, but not always), with the relative major chords used as color chords.

As long as the chords are made up of notes from the C scale, it is impossible for the 3rd of any chord to be "out of key." There are a number of so-called "perfect" intervals. 4ths, 5ths, and, of course octaves. But the 3rd (major or minor) of all seven triads available for harmonizing the key of C are—by definition—within the key.

And the same situation holds for all other keys. All chords built from notes of a key's scale, of necessity, are within the key itself.

I'm not making any of this up. My credentials include three years at the University of Washington School of Music (I was the first student admitted as a guitar major, 1957), and I also spent two years at Seattle's Cornish School of the Arts, also as a guitar major. And I have been teaching classical and folk guitar for approximately fifty years, and part of the instruction I give is in music theory (which includes, of course, scale and chord structure).

As I have said many times here on Mudcat (I sometimes feel like King Canute trying to order the tides to recede), two notes—an interval or "dyad"—is not a chord. It takes a minimum of three different notes to constitute a chord. That, too, is by definition.

Many folk musicians who have never taken any formal musical training tend to use the term "chord" rather loosely, generally referring to a finger-shape rather than the actual notes involved.

I would suggest getting a good book on music theory, especially one with a good glossary, to use as a reference.

Don Firth