http://www.perseus.tufts.edu/hopper/text?doc=AG+137 wrote:[*] b. Instead of cardinals, to express simple number, when a noun plural in form but usually singular in meaning is used in a plural sense: as, bīna castra, two camps ( duo castra would mean two forts). With such nouns trīnī , not ternī , is used for three: as, trīna (not terna ) castra, three camps; terna castra means camps in threes.
Ok, so to be clear 'trini' is an alternative form of the distributive 'terni' when a noun plural in form but usually singular in meaning is used in a plural sense.
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