So, using MTed's info:

Tonic is the first scale step--
Supertonic is the second
Mediant is the third
SubDominant is the fourth
Dominant is the fifth
Submediant is the sixth
Subtonic is the seventh


So in terms of I, IV, and V, that's this:

In the Key of C--the Chords are:

I Tonic:             C major
SuperTonic:    D minor
Mediant;         Eminor
IV SubDominant: F major
V Dominant:       G or G7
SuperTonic:    A minor
SubTonic:       B diminished


Three chord major key songs, are often C-F-G7--which is Tonic Subdominant and Dominant--[I, IV, V]


And by Stu's "sounds crap" method:

Subject: RE: Chord Req: Chord Search Help
From: s&r - PM
Date: 13 Apr 04 - 02:54 AM

crude way of finding chords that work

1. Find the key (usually the last note, or the one that the tune feels happy to finish on).

2. Play the major chord (I) on that note.

3. Sing the song slowly while playing the chord - when it sounds crap, play the V chord - continue till that sounds crap, and change back to I

4. When both sound crap, try the IV chord

this works crudely for songs that aren't minor or modal. It won't give the best chords but it will give you practice at finding them.

Stu



~S~