The definition of an acronym is a word formed by initial letters that can be pronounced as a separate word.
Examples of acronyms are "Laser" (light amplification by stimulated emission of radiation), "Nato" (North Atlantic treaty organization), and "Ram" (random access memory).
However, "TLA" is not pronounced as a word. Each letter in it is pronounced separately, as as such it's not an acronym. If you want you can call it a three letter abbreviation, but to be more specific it's really an initialism – but that would change the abbreviation from TLA to TLI.
The Guardian style guide also recommends that acronyms are to be written using an initial capital (e.g. Nasa, Nato, Unicef), and that initialisms are to be written in all-caps but without full stops and spaces (e.g. BBC, CEO, IMF).
The lines blur, of course, when you get abbreviations such as VAT (value added tax) which can either be pronounced as individual letters or as a word.