Taken from the
Eurocontrol ACAS FAQ:
What is the status of the development of ACAS that will issue both horizontal and vertical Resolution Advisories?
ACAS III - Airborne Collision Avoidance System that issues RAs in vertical and/or horizontal directions is also referred to as TCAS III and TCAS IV. It has not been implemented and it is unlikely to be in the near future. ICAO SARPs for ACAS III have not been developed. Currently, there are no plans to proceed with such a development.