Just as there are root 6 and root 5 chords, there are also moveable chords which have their root notes on the 4th, 3rd or 2nd strings. Shown below is a D form root 4 major chord shape, and a progression which uses this shape for all three chords by moving it along the fretboard. This shape is difficult to play at first, so be patient with it.
A power chord can easily be created from the above shape by leaving out the note on the first string. The chord can then be played with two or three notes. Here is an example.
The most common root 3 chord is a 3 note chord played with either the first finger or the 3rd finger and covering the 2nd, 3rd and 4th strings. Practice alternating between the first and third finger bar as demonstrated in this example.