It should be pointed out that Three Letter Agency is, itself, a TLA (Three Letter Acronym). TLAs also include the FBI, the FCC, the IRS, etc. These agencies are also known collectively as "the initials".
Typically, a Three Letter Agency is secretive, and people commonly suspect them of destroying our freedoms. The right-wingers think they're dirty commies, the left-wingers think they're evil fascists.