What I found at a luthier's site is that this is a guitar strung with 3 pairs (that would explain the name tres), tuned as follows:
gG cc Ee
I guess this is a perfect 4th and a 6th interval tuning, eh? Site says "upper case denotes a wound string".
But, another more history-oriented site shows drawings of both Puerto Rican and Cuban tres instruments, and refers to the Cuban style as "pear-shaped". BUT...it also says that "adapted acoustic guitars are the most commonly seen form of tres". This suggests the term is very, very loosely applied--the drawings indicate that 3 pairs of strings is not a requirement for calling an instrument a tres--at least not today. At least one instrument that is called a tres looks more like what others call a cuatro, and to make this even more confusing, the lonestar site indicates a cuatro has FIVE strings (or 5 pairs).
Golly, my Spanish sucks but "cuatro" would certainly hint at four, wouldn't ya think? I guess this is why music is an art, not a science.
http://www.lonestarguitars.com/
Edited By Bongo Boy on May 15 2002 at 19:52