I fell in love with the tres on holiday in Cuba. This instrument is very common there – often played in a trio with a regular nylon string guitar and perhaps a bongosera (- as in this picture of me enjoying a Mojito)!
It really cuts across in a band and I have found it an interesting instrument for recording work.
It has three courses (groups) of two strings each for a total of six strings. From the low pitch to the highest, the principal tuning is in one of two variants in C Major, either: G4 G3, C4 C4, E4 E4 (top course in unisons), or more traditionally: G4 G3, C4 C4, E3 E4 (top course in octaves).
Haven’t got any recordings of me playing it yet but here is some holiday video so you can hear what one sounds like:-