Hello Babs -
It's difficult for me to give you a straight answer as to whether I'm tuning down to CEG (or DF#A) from a Dadd tuning. The dulcimer I almost always use for 1-3-5 playing is my Sunhearth hourglass, which has a 27+5/8" scale length. I only have 3 strings on it, so I don't use doubled treble strings, and, as on all my various dulcimers, I use it for a number of different tunings. Amazingly - with the same set of strings - it works well in EBe, DAd and all the way downwards to AEA (and the various modal tunings within the same range).
You mention EG#B tuning. I use this quite a bit - it works really well.
Skip is quite right to say that 1-3-5 is no good for noter-drone playing (which I love), nor is it particularly helpful for nearly all of the folk-based music which most MD players want to play most of the time. Where it really comes into its own is opening the door to a more chromatic range of repertoire (show tunes, standards, some light classical etc.) than the MD can otherwise cope with.
Just in case you're interested, there are a couple of 1-3-5 pieces in the sound files on my FOTMD page, both in DF#A. They are "The Old Rugged Cross" and "We'll Meet Again" (a Second World War song which is enduringly popular here in the UK.)
