Keepa’s angle on this story is, naturally, historical.
The Tribune article quotes what has become the accepted history of these three (now four) missions:
This mission first was coined by late LDS President Spencer W. Kimball in the 1980s and since then has been repeated as a mantra by the church’s more than 13 million members.
This dating of the statement refers to Pres. Spencer W. Kimball’s April 1981 conference address A Report of My Stewardship, wherein he said:
My brothers and sisters, as the Brethren of the First Presidency and the Twelve have meditated upon and prayed about the great latter-day work the Lord has given us to do, we are impressed that the mission of the Church is threefold: