The three most important types of chords are major, minor, and dominant, as mentioned several times elsewhere. Each chord type is defined by the relationship between four notes: the root, the 3rd or the minor 3rd, the 5th, and the 7th or the minor 7th. The root and the 5th are the same for all three types of chords, but the 3rd and the 7th are not. In a dominant chord, the 7th is flattened, and in a minor chord both the 3rd and the 7th are flattened.
However, the most pure sound is made by triads which contain the root, the 3rd or the minor 3rd, the 5th, but not the 7th or minor 7th. Thus, there are two types of basic triads: major, which also fits dominant chords, and minor.
The character of a triad can be changed by using a bass note that is not part of the triad. By now you should know that a major triad over the 6th in the bass produces the parallel minor (in the case of C it is A minor 7). Each example in the section on major triads include a bass line using notes from the triad (1-3-5) and occasionally the 6th in order to produce the parallel minor. The sound files are rendered by Guitar Pro 6 and encoded in high-quality mp3 format (192kb/s), and they also contain a click track. The bass line is in the left channel, the guitar is in the right channel, and the click track is in the center. The tablature is available in png, pdf, and gp6 (extension gpx). As with single-note runs ear training is important so listen closely to the chords and try to make yourself familiar with the sound of the parallel chords in particular. In terms of rythm the examples are quite conservative, and they are all in 4/4 within a relatively narrow range of BPM. Their purpose is to give you an idea about the variations you can get out of three notes only.
A crucial performance requirement is to prevent unwanted open strings and fretted notes from ringing. When you brush across a number strings, make a habit of consciously stopping the pick before it moves too far. If possible you should mute adjacent strings with the left hand the palm of your right hand as an extra safety precaution. Triads on the top three, or bottom three, strings are generally desirable because there you at least only have to worry about the unwanted strings on one side of the string group you are targeting.