The term 'three-letter combo' is a reference to a type of word puzzle known as a trigram. A trigram is a three-letter combination of letters that can be used to form words. The goal of the puzzle is to find all the possible words that can be formed from the given set of letters. Trigrams are often used in crossword puzzles, where the player must find the correct words to fit into the grid. They can also be used in other types of word games, such as Scrabble and Boggle.