Arpeggios form a neat middle-ground between chords and scales – you’ll be voicing chords in a manner similar to playing a scale. Today we’re going to cover the three different triads that appear in the Major Scale. A Tonic Triad, as you should know, is built by stacking thirds in your given scale, so the formula for each is as follows:
Major:              R          3        5
Minor:              R        m3      5
Diminished:     R        m3      b5
In each of these we start with our root note, (the 5^th fret on the E string), and then play the third. In the case of A Maj this is the 4^th fret on the A string, but the 8^th fret of the E string for the other two. Our fifths are on the 7^th fret of the A string, but flattened to the 6^th fret for A Dim. All our Octaves are on the 7^th fret of the D string.
We’re playing all of these arpeggios in A, which should allow you to see the differences between each triad. As with scales, it’s important to keep to the one-finger-per-fret rule.

           Well, now what? I’d suggest trying out each of these in various positions on the neck, remembering that they are moveable shapes, (you can play them rooted on the A string as well).
Once you’re happy you can move them around a bit, try applying them to the Nashville Numbering System and see if you can play arpeggios for every chord within a given key using both the E and A string as roots.