3-digit phone numbers like 411, 911, 511 etc.

411 / 511 / 911 are nation-wide reachable US phone numbers
911
EMERGENCY number. A dispatcher will direct your call. Police, Fire Dep etc. 
Free - no money / coins needed at public phones.

411
Directory assistance / service. Provides phone numbers and usually addresses.
Cost: varies from state to state. In New York City free.
Confusing may be that some states/cities require to dial a 1 before 411. Better you ask people before dialing.
A number of private companies offer a directory service under different numbers. Charges vary.
 
511
Under this number you get traffic information nation-wide.
This service is called ITS (Intelligent Transportation Systems).
511 Travel Info is a project by the Department of Transportation - the states are responsible for implementation.
So at this point the service might not be available in every state.
Charges: n/a