Complex sentence: Harris, along with Oliver Cowdery and David Whitmer, was chosen as one of these Three Witnesses, although Smith clearly indicated that Harris's experience in seeing the plates occurred separately from that of Whitmer and Cowdery.
Split into simpler sentences: Harris, along with Oliver Cowdery and David Whitmer, was chosen as one of these Three Witnesses. Stephen Burnett to Lyman E. Johnson, 15 April 1838, in In 1839, Smith clearly indicated that Harris's experience in seeing the plates occurred separately from that of Whitmer and Cowdery.