Walter M. Miller, Jr.: A Reference Guide to his Fiction and his Life is a book compiled by William H. Roberson, which may be previewed at books dot google dot com. I've pulled up my preview in a separate tab and am copying here what the pages say.

The short story, "A Canticle for Leibowitz," was published in
The Magazine of Fantasy and Science Fiction, no. 8, dated April 1955, pages 93 - 111.

This first story, expanded and revised, became
Part One, titled "Fiat Homo," of the later book.

The novella, "And the Light is Risen," was published in
The Magazine of Fantasy and Science Fiction, no. 11, dated August 1956, pages 03 - 80.
This novella, after revision, became
Part Two, titled "Fiat Lux," of the later book.


The novella, "The Last Canticle," was published in
The Magazine of Fantasy and Science Fiction, no. 12, dated February 1957, pages 03 - 50.
This novella, when the later book was published, became
Part Three, titled "Fiat Voluntas Tua."