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If I put capo on the 3rd fret of my guitar, what would be the key that I'm playing in? What would be a chord progression in that key?
Whatever key you were playing in before the capo, add three semitones. C major becomes E♭ major, and so on. A minor becomes C minor, and so on.
If you played a C chord shape with your capo on the third fret, then that would sound as an Eb chord.