[Harp-L] Key of ? Notes to play?

Ok question the song is in the key of C so 1st position the notes CEG? So could you use these 3 notes for a song & be ok? 
If you play 2nd position G would the holes be GBD? Could you use these 3 notes and be ok?
I hope to talk to Matthew Smart tomorrow by phone on this one.
Trying to learn the flow of the notes to the key of the song. I hope that is the right way to say it. Just trying to learn. Thanks Tony Stephens

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