Most TLAs (Three Letter Acronyms) are pronounced by spelling the letters and IIRC are properly called "initialisms" rather than acronyms so I guess it should really be TLI not TLA but I've never seen that designation either.
I suspect that this is because we're somehow conditioned to expect unfamiliar words to have at least four letters and/or that we're used to speaking the letters when there are three or less because we do that with people's initials.
For example, I've never heard TSA, FBI, USA, DOJ, PLA, LOA, PDA, POS, LPG, LNG, ETA, ETE, QNH, or NVG spoken as a word just to name a few TLAs (or TLIs). Of course, some of those are kinda hard to pronounce as words but that doesn't stop us from doing that with some equally difficult 4 and 5 letter combinations where we often just insert a vowel or two to make it sound like a word.
OTOH (one of the few four letter "initialisms", BTW) JAG appears to be generally spoken as a word and (at least according to the TV show) NCIS isn't. Go figure.