Combinatorics
The number of possible three-letter abbreviations using the 26 letters of the alphabet from A to Z (AAA, AAB ... to ZZY, ZZZ) is 26 × 26 × 26 = 17,576. Another 26 × 26 × 10 = 6760 can be produced if the third element is allowed to be a digit 0-9, giving a total of 24,336.
In English, WWW is the longest possible TLA to pronounce, typically requiring nine syllables. Although in written English it is an abbreviation, in spoken English it ordinarily uses more syllables than the words it is abbreviating.
Read more about this topic: Three-letter Acronym