The 3PX line was marketed exclusively in the Outer Rim Territories, and due to the diversity of that region, had a less Human design than the mainline 3PO units. The 3PX-series could not match the 3PO's popularity, in large part due to its harsh, angular face, similar to that of the later RA-7 protocol droid model. The series thus enjoyed only a limited production run.
One of the most infamous 3PX units was C-3PX, converted into an assassin droid by Darth Maul and later refitted with the chassis of a 3PO unit by Raith Sienar.
Darth Vader used one of these droids, K-3PX, in his search for Luke Skywalker on Gamandar.