well, technically a power chord only has two different notes. such as:

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-2-
-0-

but you can play it with three notes like so:

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-2-
-2-
-0-

because that second "2" on the 4th string is an E, which is the same exact note as the "0" on the 6th string, so you are playing a root, a fifth, and then another root. so technically it's only two different notes because you're playing the same note in two different places.

now, if you add a third different note, it's not really a power chord anymore, it's a triad. which is what most chords are.