Three letter acronyms

BBC, ABC, DNA, RNA, ATM, USD, INR, GBP, LCD, TLC etc etc.

What is common in all this?
They are all Three Letter Acronyms(TLA!). So, Why are the three letters so popular? Why can't it be two-letter or four-letter?

Let us take some examples: If we look in Medicine which has a long list of acronyms; 3 letter ones stand out....For eg: CAD(Coronary Artery Disease), TOF( Tetralogy of fallot or Tracheo-esophageal fistula etc) or FSH ( follicle stimulating Hormone) or DNA, RNA etc

In Clinical-Genetics there are a group of disorders called "Trinucleotide repeat disorders" (also known as trinucleotide repeat expansion disorders) caused by trinucleotide repeats (like CAG, CGG etc) in certain genes exceeding the normal, stable, threshold, which differs per gene.

Countries, Cities, Airports all have popular 3 letter acronyms like USA, NYC, JFK, TRV, DBX etc etc. If we look at the names of politicians; its AKG, EMS, PKV etc or political parties---BJP, CPM, CPI, or INC. The list is endless.

Is this because our brain has some sort of inherent tendency to remember and recollect acronyms especially 3-letter acronyms? I looked in Oxford dictionary and also 'Googled' this: In English language, it says, as literacy rates rose, and as advances in science and technology brought with them more complicated terms and concepts, the practice of abbreviating terms became increasingly convenient especially from the mid 20th century. I am not sure whether this trend is also seen in other languages.

Its interesting to think about the Malayalam saying in this context- ഒന്നില്‍ പിഴച്ചാല്‍ മൂന്നു!!!