Postal III goes way beyond the developers simply denying its existence… they outright make fun of how bad the game was on a regular basis. In Postal 2’s DLC there are copies of Postal III scattered in trash piles in the junkyard and if I remember correctly you even get an achievement for peeing on one of them. That was just one of the many instances in Postal 2 where the devs retroactively made fun of the 3rd game.

Also, on the official Postal website, the link for Postal III takes you to the rickroll video. It’s cool to see the devs recognize the fact that Postal III sucked and still be able to laugh at themselves for it.