The term ONE OF THREE IN A MORSE 'S' is solved by the three-letter word DOT. This phrase refers to Morse code, a system of communication developed in the early 19th century by Samuel Morse. Morse code uses a series of dots and dashes to represent letters, numbers, and symbols. In Morse code, each letter is represented by a combination of three dots and dashes, with one DOT representing the letter 'S'. Therefore, the answer to the crossword clue is DOT, as it is one of three elements that make up the letter 'S' in Morse code.