What is the basis for the doctrine of the Trinity in the Book of Mormon? The Community of Christ – formerly the "Reorganized Church of Jesus Christ of Latter Day Saints" – is a rather unique branch of Christianity in that it stems from the LDS Church, recognizing the Book of Mormon, but is also Trinitarian in its beliefs.

It seems to follow from this that they must conclude that the Book of Mormon affirms the doctrine of the Trinity and does not teach eternal progression like the LDS church teaches.

My question, then, is what is the Book of Mormon basis in support of the doctrine of the Trinity?
With a fair bit of searching, I could find only one source associating the Community of Christ's belief in the trinity with verses from the Book of Mormon. The verse is 3 Nephi 11:36:


  And thus will the Father bear record of me, and the Holy Ghost will bear record unto him of the Father and me; for the Father, and I, and the Holy Ghost are one.


In order to join the National Council of Churches (NCC), The Community of Christ had to make several concessions, including reducing it's reliance on the Book of Mormon. A 2010 NCC report made the following statement about the Community of Christ's belief in the Book of Mormon:


  Leaders of the COC... affirmed without qualification that the Bible 
  is the foundational, authoritative Scripture of the church.  They acknowledged that “the Book of 
  Mormon is in our DNA,” and suggested that it confirms God’s revelation in Jesus Christ as testified in 
  the Bible.  But it is not, in any sense, equivalent to the Bible in the life of their communion.
  Subscription to its teaching is not required for membership or ordination.  While the Book of Mormon 
  is sometimes used in worship, there are parts of the COC that seldom refer to it. 


Considering that the Community of Christ has made the Book of Mormon optional, I doubt they feel the need to find support for any of their doctrine in it beyond general support for Christ.