Still can't find when it was all placed in a triangle but I did find the original paper of when ease of use was first published in 1975. I know it's not what you were looking for but still interesting all the same. I think you'll be fine to use a secondary source in this instance though.
h) Psychological acceptability: It is essential that the human interface be designed for ease of use, so that users routinely and automatically apply the protection mechanisms correctly. Also, to the extent that the user's mental image of his protection goals matches the mechanisms he must use, mistakes will be minimized. If he must translate his image of his protection needs into a radically different specification language, he will make errors.
Saltzer and Schroeder, The Protection of Information in Computer Systems (virginia.edu)
Manuscript received October 11, 1974; revised April 17. 1975. Copyright © 1975 by J. H. Saltzer.