A tres carries six strings in three courses, and the two outer courses are tuned in OCTAVES rather than unison — the registry's own entry for this instrument records it: G and C in octaves, E in unison. That is not a detail. It is the reason a single-note figure on a tres sounds like two instruments, and it is why the octave string of a course is the one whose death you notice first.
Which course is which
Working from the registry's tuning, the three courses are G, C and E. The G and C courses each pair a string with its octave; the E course is two strings at the same pitch. So of the six strings, two are octave partners, two are their lower halves, and two are a unison pair. When a string dies, ask which of those three jobs it was doing before you decide how urgent it is.
Sets, and the shortage of them
Purpose-made tres sets exist and are hard to find outside Cuba and a handful of specialists. The common workaround is assembling a set from individual guitar strings, which works because the gauges involved sit comfortably inside the range a guitar set covers. What matters is not the brand but the relationship: the octave string of a course wants to be light enough to reach its pitch without excess tension, and the lower string heavy enough not to sound thin beside it.
Whatever you assemble, write down what you used. A tres set is a recipe, and the second time you string it you will want the same one — or a deliberate change from it, which is a different thing from a guess.
Changing one string or both
Change the pair, not the string. A fresh string beside a year-old one is louder, brighter and out of tune with its partner within a day, and on the octave courses the mismatch is unmistakable. Changing both keeps the course sounding like one voice, which is the entire point of a doubled course.
What a dead octave string costs
Play any figure from the guajeo pages with one octave string dead and it flattens into a single-line melody. The bright upper octave is what makes a montuno cut through a band without volume, and it is the first thing to go dull. If a tres has started sounding "quiet" without anything obvious being wrong, this is usually why — long before anything is worn out enough to look wrong.