Does anyone know when FIBA introduced three-person mechanics? And is this going to be an occasional thing or the norm?

At the FIBA Central Board's meeting in May of 1999, three man mechanics were accepted for implementation after the 2000 Olympic games.

NCAA three-man mechanics will now be used for all Olympic Games and World Championships.

Also three men can be used for any FIBA game where the governing body of the competition in question wishes to do so.

FIBA
 

Three man mechanics using the NC2A method were used for the first time by FIBA at the ParaOlympics.

Three man officiating is also used in Europe via the ULEB Euroleague.