AMD is lagging behind Nvidia when it comes to adaptive sync. Nvidia’s G-Sync technology works on both multi-GPU and multi-display systems, whereas AMD has not even released a driver yet that supports Crossfire builds.
AMD’s FreeSync is therefore currently exclusive to single GPU-builds. AMD said that they would release a driver at the end of April that would allow support for Crossfire builds, but the company has delayed the release. Both triple-display and multi-GPU systems are gaining more and more popularity, so hopefully AMD will release a driver pretty soon that allows FreeSync to work with Crossfire builds.