The first question is not a price, and it is not the body. Plenty of instruments sold as a tres are a six-string guitar body with a tres neck and bridge fitted, and plenty of those are excellent. What makes it a tres is three COURSES of two strings each, tuned so that the outer courses ring in octaves — and a neck and bridge built to hold six strings in three tight pairs.
Purpose-built or converted
A purpose-built tres has a narrower body, a shorter scale, and a bridge spaced for three pairs from the start. A converted guitar has a wider body and a longer scale, which gives more string tension and a bigger sound but asks more of the left hand at speed. Neither is the "real" one; converted instruments have been played professionally for as long as the tres has existed. The question is whether the conversion was done properly, and that is a question about the nut and the bridge.
What to check, in order
Sight down the neck for a bow — six strings in three courses pull hard, and a neck that has already given up will not be rescued by a setup. Then look at the nut: each pair of slots should be evenly cut and evenly spaced, with the two strings of a course close enough that a pick strikes them as one event. Uneven pairs are the single most common fault in a cheap conversion, and they make every figure on the guajeo pages harder than it should be.
Then turn all six tuners. Six cheap tuners on three courses go out of tune against each other, not just against the room, and a course that is out of tune with itself sounds broken rather than flat. Finally press each course at the twelfth fret and listen for the octave — on the G and C courses you are listening to two different octaves land together, which is the sound you are buying.
Second-hand
Outside Cuba a used tres is often the only tres, so the choice is frequently made for you. Check the same four things and ignore cosmetics entirely. A dull instrument with a straight neck and clean nut slots is a good buy; a bright one with a twisted neck is a wall decoration. Bring something you can play — even one shape from the chord library — and play it before you decide, because a tres tells you within a minute whether the action is fightable.
One thing not to assume
Guitar habits transfer badly here. Six strings in three courses are not six independent voices: you are fretting pairs, and a shape that mutes half of one course sounds like a mistake rather than a voicing. If you are coming from guitar, expect the first week to be about the right hand striking pairs cleanly, not about learning new shapes.