I, IV, V chords

Last night we went to a concert.  There were guitars and ukuleles. At intermission I mentioned that I thought the kids were great.  A friend responded: They are great. But remember like most you hear they are only playing the I, IV and V chords.  Okay, my dulcimer is tuned DAd.  What are the I, IV and V chords?  If the tuning is different are the I, IV and V chords different?