The 3rd prohibition is that we are forbidden from making an idol, even for others to serve, and even at the request of a non-Jew.

The source of this commandment is G‑d's statement (exalted be He),1 "And do not make gods for yourselves."

In the words of the Sifra, "[The verse,2] 'And do not make gods,' means that you may not even make them for others." [In Sifra] our Sages said, "One who makes an idol for himself transgresses two prohibitions," i.e. the prohibition of making an idol with his own hand, even for others, as explained in this Third prohibition; and also the prohibition of acquiring an idol and having it made with his permission — even if someone else made it for him — as we explained previously in the Second prohibition. Therefore he is punished with two sets of lashes.3

The details of this mitzvah, as well as the previous one, have been explained in tractate Avodah Zarah.