ADS-B is currently approved & used for 5NM separation. I don't think 3NM has been approved yet but is being worked on - gathering data to confirm accuracy & reliability in real use.
Aircraft equipment can display ADS-B information for nearby aircraft, although what is displayed depends on the capabilities of the equipment. Controller needs & pilot needs aren't necessarily identical.
To clarify, the ADS-B equipment is mainly co-located with VHF radio outlets as power, data links & a tower are available.