Code Three is an emergency code that is activated when an infiltrator, such as a Goa'uld, is already inside Stargate Command. It is activated by declaring a code three through the base intercom. In 2001, Major Samantha Carter activated it because the Goa'uld Nirrti was inside the base collecting information on Cassandra Fraiser whom as she was trying to produce a "Hok'tar". (SG1: "Rite of Passage")