6 string dulcimers have been around since before most people on this page has been living, however until recently most everyone had given up on trying to "jam" with dulcimer along with 5 string banjo, flattop guitar, or bluegrass band for one reason only and that was because no one else in the band or audience could hear the dulcimer.
The 6 string dulcimer is tuned just like a 3 string except the strings are doubled and each double string is actually tuned as one string (in unison) therefore the dulcimer is much louder (it can be heard). Also you can "cross pick" on it and make some nice music.
If you ever play a Wild Cherry,6 string dulcimer with Grover Roto-Matics tuned to a High Bass Ionian pitch you will throw rocks at the others. All of this is not to say that a 3 or 4 string dulcimer is not a nice instrument (they are) but lets not talk bad about the 6 string dulcimer.
Thanks I knew you would understand.