I found the porn clip. A more-or-less normal-looking young woman with three relatively-narrow breasts. I am uncertain whether the third breast was covered in living tissue, or was a prosthetic. It certainly looked similar to the others. But I was reminded that the typical mammalian pattern is to have pairs of breasts balanced on either side of the body centerline.
In the SF movies, triple breasts served well to emphasize the alienness of the person, but in the clip, the rest of the young woman’s body was normally bilaterally symmetric. So the third breast definitely stood out, so to speak.
My guess is prosthetic. For a few seconds there appears to be an odd horizontal wrinkle line near the collar bone – perhaps the top end of the prosthetic.
I know it’s been a long time but I recently learned that reports of prosthetic use also killed the careers of aspiring porn stars Zena Fulsom and her “sister” Cindy. Seems they did have fake breasts but not in the usual way, they reportedly were using prostheses which fit over their natural breasts. I suspect farang ding dong models have used the same method.
Do not ask Google about Zena, Cindy, or farang ding dong while at work!
To all of you people who keep citing Total Recall and Hitchhikers Guide, you ought to know that triple-breasted and multi-breasted characters are pretty common in SF literature and especially art, and long predate those cases
Go look up “Multibreast” on deviantart and you’ll be swamped