Triads are three-note chords. What you may not have realized before is that even chords like the common six-string G chord (320003) is just a three note chord. Those notes are G, B and D, except that we have some duplicates showing up in different octaves, thanks to playing three notes across six strings.
Well, if we cut off a little fat we can play that same chord using just three strings. In today's lesson we're going to do that using strings 1-3. Think of your E major chord shape, and then bar it at the third fret (355433). That gives us a G major. Now, pay attention to what notes you're fretting, and re-arrange your fingers so you're just playing the top three strings like this: xxx433.
That's a G major chord, also called a G major triad.
The G in that chord is actually found on the first string, so I'm calling this a "Root 1" shape. That root note is the key to moving this chord elsewhere on the fretboard, so if you move that shape up to the 8th fret, you'll find a C on the first string, which means that shape produces a C major triad. (xxx988)
Okay, now let's look at minor triads.
Think of your open E minor chord, then move it up to the fifth fret and bar it. That should look like this: 577555.
That's an A minor chord. Now we can pull the same trick and use only the top three strings, and we'll still have an A minor chord: xxx555.
Well, that shape can be moved around as well, and because the root note is also found on the first string, we use that string to locate our root notes.
So, if you move that shape to the 7th fret (xxx777) you get a B minor triad.
In future lessons we'll look at shapes you can use on these same strings, but using root notes coming from the other strings as well.
If you're interested in learning more about triads, checkout my Secrets of Tasty Riffs & Solos course; we dig into triads a TON there!