Update: Following our article, Deadline has since corrected themselves with note Avengers 3 is currently untitled.
In Deadline's report about Brie Larson as Captain Marvel, they make mention that the character may first appear in a different Marvel franchise "possibly Avengers 3 (formerly Avengers: Infinity War Part One)."
The report also states Captain Marvel may appear in Guardians of the Galaxy 2.
Regarding the reason for the change to Avengers 3, we can speculate it's for branding and marketing purposes, and it might even signity the first Infinity War movie is set more on Earth.
The Avengers 3 has a May 4, 2018 release.