Final answer:
The stomach is the part of the GI tract that has three sets of muscles, which include an inner circular layer, an outer longitudinal layer, and an additional oblique muscle layer for churning.
Step-by-step explanation:
The part of the GI tract that has three sets of muscles is the stomach. The muscularis externa layer in the stomach is made up of an inner circular layer, an outer longitudinal layer, and an additional third layer known as the oblique muscle. This additional muscle layer equips the stomach for its churning function, which is important for mechanical digestion. By contrast, the small intestine and large intestine each have two muscle layers; however, the large intestine's longitudinal muscle is divided into three bands called the tenia coli.