Three Letter Agencies are a general category of "spook" agencies, which defend America, but generally act mysteriously. A knock on the door at any hour from any of them should make you want to run for the back door or jump out the nearest window, except it won't help since they're already watching it.

Some are likely to make you disappear. Some may just want to question you. Some, you'll never know about, but they're already watching you. It doesn't matter that you may not have heard of some of them, they already know who you are, who you've dated, and what you ate for breakfast.

ATF - Bureau of Alcohol, Tobacco, Firearms and Explosives

CIA - Central Intelligence Agency
DEA - Drug Enforcement Agency
DHS - Department of Homeland Security
DIA - Defense Intelligence Agency
DOD - Department of Defense
DOE - Department of Energy (yes, them too)
FBI - Federal Bureau of Investigation
INR - Bureau of Intelligence and Research
ISR - Air Force Intelligence, Surveillance and Reconnaissance Agency
NRO - National Reconnaissance Office
NGA - National Geospatial-Intelligence Agency
NSA - National Security Agency
ONI - Office of Naval Intelligence
TFI - Office of Terrorism and Financial Intelligence

USSS (United States Secret Service) falls into this category, even though it has 4 letter in its acronym.

the thing that you don’t have when you’re phones percentage is over 20

The federal agencies comprised of three letters.