Hi, Phil. I can't find my copy of 'Rounding the Horn.' Give me a while to get back to you on the chords.

If you're writing three-part harmonies, you must know your chords. Let me pass on a couple of thoughts.

When selecting chords, pay attention to the difference between mere dissoance and true discordance. Suppose we have a song in the key of G. Suppose a measure has these notes:

G B D E.

A G chord might do, even though the E is not in it. You might consider the E a passing note.

But if the measure has an C in it, watch out. The C is going to clash badly with the B in the G chord. (discord) It will also clash somewhat with the D (dissonance.) I believe that in folk music, you can get away with dissonance, but people will not accept discord. To me, discord sounds like jazz and smells like booze.

To deal with the note C, you might play a C chord, a D chord (producing a D7) or an Am. These are all chords that have a C in them. All you can do is try them and see what you like. And as somebody observed already, you are free to use one chord one time and a different chord another time.

Another approach is to re-write (gasp!) the timing and make the C note so short that it doesn't get on people's nerves.
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When a melody is moving up or down the scale, no chords are going to go with it. I often play just the tonic or the tonic plus the fifth of some chord when that happens. It doesn't have to be the root chord, it could be some other chord.

I don't believe the common saying that folk music uses I IV and V chords. The old-timers knew that the scale went "do re me fa so la ti do." Knowing that, how hard is it to ask yourself what chord is based on each of thdse? Answer: not hard at all. So in the key of D, I might use:

D
Em
F#m
G
A
Bm

I gotta admit that I rarely use the chord based on the seventh note, the C#m. Maybe I ought to.