The phrase "When tripled, an ellipsis" refers to the fact that an ellipsis consists of three dots. When you triple a single dot, you get an ellipsis, which is represented by three dots in a row ("..."). Thus, the answer to this clue is "DOT" — a single dot tripled becomes an ellipsis.
An ellipsis is used in writing to indicate omission of words, pauses, or unfinished thoughts, and it is composed of exactly three dots, also called suspension points or dot-dot-dot.
