Tootler, in a minor chord, the third is only 3 semitones (or half-steps) apart ie A - C. That's called a minor 3rd (or -3). When played together with the perfect fifth it produces the sound of a minor chord ie A-C-E.

In a major chord, the third is one semitone larger ie A - C#. That's called a major 3rd (+3). PLay it together with the perfect fifth and you hear the distinctively different sound of the major chord ie A-C#-E.
So, the only difference between major and minor chords is the size of the third; 3 semitones for the minor, and 4 for the major.

So in other words, the third is "lowered" in a minor chord and "raised" in a major, not the other way around. It's the same in any key ie C-Eb-G is a C minor chord, and C-E-G is the major.

Is this what string quartet players do instictively? If so is it really Pythagorean intonation which has pure fifths and fourths but considers the third a dischord?

String quartet players are most often playing from a written score, with the key signature (ie any sharps or flats necessary to produce the major or minor key desired) clearly indicated at the beginning of each line of music. All 24 major and minor keys have their own unique key signature -- so producing the correct 'size' of third - or any other note/interval in the piece - is not instinctive, but learned.

I don't think thirds are discords though. They sound pleasant enough, don't demand "resolution" of any kind. They're just highly mutable! :-)

Now, a +7th (C-B) or a diminished 5th (C-F#)? Those are discords! (Not meaning to diss anyone, just my personal taste).