Ameet - There is no requirement in Australia (yet) for 3nm separation using ADS-B because all of our busy TMAs are equipped with radar which is currently being upgraded to Mode S. Radar (SSR) is still the preferred separation sensor because it utilises transponders with which aircraft are almost universally equipped. We did however recently commission a Wide Area Multilateration (WAM) system in the Sydney Terminal Area to augment radar coverage and provide PRM to Sydney's close spaced parallel runways. It is approved for 3 nm separation and PRM. (The accuracy throughout the coverage area is generally 150 metres, reducing to 50 metres on long final and 30 metres on short final for PRM)
Another reason for not having 3 nm approval for ADS-B is that ADS-B equipment will only be mandatory above FL290(?) so the terminal areas will still be equipped with non-ADS aircraft. These can be tracked by radar and WAM because they track the transponder and do not require the transmission of position information.
The ideal situation would be for all aircraft to be ADS-B equipped so that ground surveillance becomes cheaper lowering the cost of ATC to aircraft owners. This has proven to be impossible even in Australia where arguably it would have been much easier than in more crowded continents. Fortunately WAM recievers can also recieve ADS-B (same 1090Mhz frequency) so the ones we are installing will still be useful later. By the way aircraft need ADS-B IN to recieve other aircraft ADS-B OUT transmissions. This is new and only just being incorporated in new aircraft.
Another use for ADS-B will be on the ground where we currently need about 15 multilateration receivers around an airport for A-SMGCS when probably 1 or 2 ADS-B receievers could do the same job.