There are 263 = 17576 possible abbreviations, and probably most of them are already used. For an illustration, follow the link below to a page of all TLAs from AAA to DZZ. If we allowed numbers, special characters, or case-sensitivity, many more TLAs could be created.
Many TLAs have more than one meaning. There are even TLAs with more than 10 meanings (for example, SDI has at least 32 meanings in the English language). Many abbreviations have more than one expansion with the same meaning. For example GCC was first 'GNU C Compiler', and later 'GNU Compiler Collection'.
In the DOS operating system, because only three-letter file extensions (usually denoting the file type) were allowed, many longer abbreviations were shortened to three letters (for example JPEG to JPG, HTML to HTM).
Many abbreviations come from the shortened names of Usenet groups. For example pra for pl.rec.anime.
Common categories of TLAs:
A four-letter abbreviation (e.g., VERA) is sometimes known as an ETLA (extended TLA).