A tres is set up around one fact: the pick strikes two strings as a single event. Spacing, action and intonation are all judged against that, and each is a different job from the same-named job on a six-string guitar.
Course spacing
The two strings of a course want to be close enough to be struck together without aiming, and far enough apart that neither rattles against the other under a hard stroke. Between courses you want the opposite: enough room that the pick can clear one pair and land on the next without catching. Most setup complaints on a converted instrument are really spacing complaints, and they are fixed at the nut and the saddle rather than by playing differently.
Action
Figures like the ones on the cha-cha-chá page move quickly between courses and punish high action immediately — not by being unplayable but by being tiring, which is worse, because it shows up half an hour in rather than in the shop. Set it as low as it will go without buzzing under the stroke you actually use. A tres played hard needs more clearance than one played lightly, so set it for your right hand rather than to a number.
Intonation across a doubled course
Six strings across three courses means intonation has to be right for both members of a pair, and the two strings of an octave course do not want identical compensation. If a course plays in tune open and sour at the twelfth fret, that is a saddle problem; if only one string of the pair is sour, it is that string's own break point. Check each string separately, then listen to the pair together — a course that is fine string-by-string and beats as a pair is telling you the two are fighting over the same saddle position.
Humidity and the rest
The tres comes from a hot, humid place and often ends up somewhere dry, which is harder on it than heat ever was. A cracked top is expensive; a case with a humidifier in a dry winter is not. Beyond that, the maintenance list is short: keep the nut slots clean, keep the pairs changed together, and do not chase tone with hardware before you have tried the free adjustments in the strings guide.