Maryrrf,

A three stringed dulcimer tuned DAD is in mixolydian D. It is mixolydian because of the relationship of the pitches of the three strings.

A three stringed dulcimer tuned CGC is in mixolydian C. Different pitches, same relationship.

Three of the strings of a guitar in DADGAD match the mixolydian tuning of a dulcimer. But then the other three strings don't. Whether that guitar tuning is called "modal" is up the the guitar playing community. The extra strings complicate the discussion.

For what it is worth, a standard banjo tuning is ADADE. Although this banjo tuning shares pitches with a dulcimer tuned mixolydian D, old time banjo players would definitely NOT call this tuning modal.