When did the Community of Christ (formerly RLDS) adopt the doctrine of the Trinity? The Community of Christ–formerly known as the Reformed Church of Jesus Christ of Latter-day Saints (RLDS)–is a schism of the Church of Jesus Christ of Latter-day Saints (LDS). They separated from the main body of the church after Joseph Smith's death, believing that the line of prophetic succession should follow Smith's bloodline. 

Since then, they have made many fundamental changes to their doctrine, the most surprising change being that they now believe in the doctrine of the trinity. 

Joseph Smith saw God the father and the Son standing side by side.


  I saw two Personages, whose brightness and glory defy all
  description, standing above me in the air. One of them spake unto me,
  calling me by name and said, pointing to the other—This is My Beloved
  Son. Hear Him! (Joseph Smith History 1:17)


Joseph Smith clearly taught that God the Father, Jesus Christ, and the Holy Ghost were individual beings, and that both the Father and Christ had bodies of flesh and bone (see D&amp;C 130:22).

To this day the Community of Christ identifies Joseph Smith as their founder. (see Community of Christ: History), and just prior to his martyrdom, Joseph Smith was quoted saying:


  "I have always and in all congregations when I have preached on the
  subject of the Deity, it has been the plurality of Gods. It has been
  preached by the Elders for fifteen years." (Joseph Smith, June 16,
  1844).


At what point did the RLDS abandon the most fundamental doctrine of the restoration and adopt the trinity?
Paul Edwards, in his book "Our Legacy of Faith: a brief history of the Reorganized Church of Jesus Christ of Latter Day Saints (RLDS)" (Herald Publishing House, 1991) indicates that the transition to belief in standard American trinitarianism occurred gradually towards the turn of the 20th century, and that the transition was complete by 1890:


In 1878 the "Reorganization accepted the inheritance of "early" Mormonism and thus identified itself with a more orthodox Christian position of God, Christ and the Holy Spirit as an infinite, almighty, and all-powerful Godhead." (p.144)
Previously-held views "simply faded away as the more orthodox Christian view of God, that of the Trinity, became the primary belief of the movement. By 1890 the plurality of gods was referred to as a 'doctrine of man.'" (p.161). 


For more detail one would probably need to dig through the old Conference papers themselves.