The string on the tres are set as the picture shows.
Notice that the out side strings are both E strings from the regular guitar. The inner strings are G strings, they are paired with the E strings. The center strings are B strings.
The tres is tuned in C concert. Once tuned when you play open strings it will be C chord = G+C+E
in your iphone app store look for pitchPerfect by a company called NCH Software. Is a free tunner that can tune all kinds of instruments perfect. Get familiar with it as it is very simple to use and set it up as UKULELE tuning. It just happens that the second, third and fourth strings in the ukulele tune the same as the tres. The Ukulele tunes in G+C+E+A
I will use guitar terminology. Begin with the E string and the G string both has to be tune to E. In the picture, they are located in your right hand side of the screen. The E string is tune to E like a regular guitar and the G string is tune to E.
The B strings: Tune both strings unison to C concert.
The last strings are also E strings and G strings. Tune them to G.
Once in tune just play slow to see how it sounds.
Note: go very slow tuning the last strings to G. The strings can break if you go too high or passing G. Tune it to E first then slow move up to F, F#, and G.
