A
three-letter acronym,
three-letter abbreviation, or
TLA is an
abbreviation, specifically an
acronym, alphabetism, or initialism, consisting of three letters. These are usually the initial letters of the words of the phrase abbreviated, and are written in capital letters (upper case); three-letter abbreviations such as
etc. and
Mrs. are not three-letter acronyms, but "TLA"
itself is a TLA (an example of a self-referencing definition).