The 1986 press release for the Deskpro 386 mentioned 386 Xenix
was planned for 1987.
https://www.latimes.com/archives/la-xpm-1986-09-10-fi-13177-story.html
"UNIX System V and the 80386 are a perfect technological match," said Bill Gates,
chairman of Microsoft Corporation, in remarks at AT&T's press conference here. AT&T
and Microsoft are developing a new version of UNIX System V for the 80386 chip that
will run XENIX System V as well as UNIX System V applications.
This is September 1987, so perhaps Microsoft's abandonment of Xenix was not as early as I had thought. Though this does imply that the Xenix port was not ready at that point, and perhaps was ultimately abandoned by Microsoft.
Intel and AT&T had ISC do a 386 port for SVR3.
"The 386/ix is based on Release 3.0, which was developed by Interactive
Systems Corp. Santa Monica, Calif., under contract to Intel and AT&T.
The code was tested through an extensive beta program managed by Intel
(with more than 60 80386 beta sites)."
Mini-Micro Systems, January 1988, p 45
https://archive.org/details/bitsavers_MiniMicroS_59292072/page/44/mode/2up
https://bitsavers.org/magazines/Mini-Micro_Systems/198801.pdf
AT&T sold rebadged Olivetti machines with SVR3 in 1987:
"AT&T 6386 WGS is today's only 80386-based system to take full advantage
of its 32-bit architecture"
https://bitsavers.org/pdf/att/6386_wgs/6386_WGS_Brochure.pdf
ISC work was also used by Microport:
"Microport Runtime System V/386 is based on a version of Unix for the
80386 carried out by Interactive Technologies for AT&T and Intel."
Microport to Ship Version of Unix for 386
InfoWorld, Volume 9, Issue 9, 2 Mar 1987, p 3
https://books.google.com/books?id=1TAEAAAAMBAJ&pg=PA3