Guitar / Tres
Guajeos were developed and played by itinerant musicians on their guitars, and the same rhythms were eventually transferred to the piano when the son became urbanised into a salon setting. Through this link, we can understand the role of the guitar and tres in salsa despite the general paucity of specific instructional material, by looking at those available for piano and violin.
The Cuban tres has three double-string courses where the outer two courses are octavadas (octaved) i.e. with strings of each pair tuned an octave apart, and those of the center or second course are unison. Tuning is a matter of preference: Gg-cc-Ee (C major), Gg-bb-Ee (E minor) or Ff-dd-Aa (D minor). Sometimes a capo is placed on the second fret of the former resulting in a D major tuning.
The most common tres to be found is adapted from a standard guitar; where three extra holes are drilled into the bridge and the nut modified to give rise to the three double-string courses. The unused holes in the bridge are filled in with wood typically from violin tuning pegs. Example string gauges are:
I use the E minor tuning on my tres, as per the three highest-pitched strings of the guitar.
What to play
A good place for the proficient guitarist to start is the material on 'AfroCuban montunos for guitar' by Carlos Campos which has music in both notation and tab form plus an audio CD to help with phrasing. Although rich in examples, it is a little light in terms of explanation. You may want to refer to Rebeca Mauleon's '101 Montunos' for a comprehensive treatment of montunos; and Sam Bardfeld's book on phrasing and soloing.
For more advice, you may want to refer to the Rhythm: Piano page of this section.
Salsa Guidebook For Piano & Ensemble by Rebeca Mauleón. (External link)
Salsa, Afro Cuban Montunos For Guitar by Carlos Campos. Book and CD package. Plenty of example progressions, a number of which would be directly translatable onto tres. (External link)
101 Montunos by Rebeca Mauleón. Contrary to the title, there aren't 101 montunos listed in order. Instead the approach is skills-based; teaching you how montunos work, how to play them, and providing plenty of case studies. A truly exceptional piece of instructional material. (External link)
Latin Violin: How To Play Salsa, Charanga and Latin Jazz Violin by Sam Bardfeld. Although targeted at violin players, it has a wealth of general information about playing in a salsa ensemble. The soloing case studies are particularly helpful. (External link)
Other references can be found under the Instructional > Other category of Descarga.com. (External link)
& Merengue Society