You will nearly always meet this word in the fixed phrase 'in triplicate', which means a document exists as three identical copies — one for each of three people or offices. It comes from the world of paper: hospitals, customs, banks and armies all used forms with carbon paper behind them, so pressing hard with a pen made three copies at once. Because of that history the phrase carries a strong feeling of tiresome official rules, and English speakers now use it as a joke about bureaucracy: 'you have to apply in triplicate' means 'the paperwork here is ridiculous', not literally that there are three sheets. There is also a verb, to triplicate (to make three copies of something), and a noun, a triplicate (one of the three copies), but both are rare — the phrase 'in triplicate' is what you actually need. Compare 'in duplicate', which is the same idea with two copies and is just as common.