Final answer:
The Arikara tribe joined the Mandan and Hidatsa tribes to become the third of the 'Three Affiliated Tribes' in central North Dakota.
Step-by-step explanation:
The tribe being referred to in the reading selection that joined the Mandan and Hidatsa people in central North Dakota, becoming the third of the "Three Affiliated Tribes," even after being decimated by disease, is the Arikara. Together, these tribes are often referred to as the Three Affiliated Tribes and reside on the Fort Berthold Reservation in North Dakota. The history of these tribes and their interactions with the United States government encompass treaties and forced relocations, as seen in the experiences of various other tribes such as the Lakota Sioux, Cheyenne, and Apache.