Why is LOO the purest spiritual source?

[isis]

Because the three of them together represented an archetypal purity of Power (Jim), Love (Carla), and Wisdom (Don).

It's interesting to note, though, that Jim wasn't even there when Ra first came through. So, I'd argue that maybe it had less to do with the specific individuals and more just to do with the energies brought to the table.

Jim: During the 106 sessions with Ra there were only three people who ever attended a Ra session besides the three of us, and in each case it was Ra’s recommendation that each entity needed not only to have the appropriate attitude in its personal means of seeking but that each person needed to be in harmony with each of us before attending any session. In Tom’s case this was achieved by Don’s explaining to Tom the meaning that the Bible, candle, incense, and chalice of water held for us as triggering mechanisms or signals to our subconscious minds that a session was about to take place and that from all levels of our being we should begin the process of purifying our desires to serve others above all else and to surround ourselves with the joy-filled light of praise and thanksgiving. The harmony that this process produced among our group, then, was much as a musical chord with which those of Ra could blend their vibrations, and upon that harmonious blend of vibrations information of a metaphysical nature could be transmitted by being drawn to those which sought it.

Those three archetypal representations of the magical personality, and a pure desire or intent to acquire truth allowed a great degree of undistorted truth to flow through them.

It's also interesting to note that they didn't even originally set out with the intention of channeling Ra. Ra first appeared, much to their surprise, when they had intended on doing a teaching session, a conscious channeling from a familiar source, for one of Crala's students.