Three Letter Acronym (or Three Letter Abbreviation - see the Acronym page for a discussion on the difference.)

An acronym (or abbreviation) with three letters. RAM, CPU, CSS, the computer industry loves these. It's probably something to do with the fact that humans can easily comprehend things in blocks of three (did you know the easiest layout for an automatic "Press 1 for sales" style phone system is to have no more than three levels with only three options each?)