Final answer:
After eating, running, and needing the restroom, the body primarily uses the digestive, muscular, and excretory systems to process food, enable movement, and excrete waste.
Step-by-step explanation:
The three body systems primarily being used after eating a McDonald's meal, going for a run, and then realizing the need to use the restroom are the digestive system, muscular system, and the excretory system.
The digestive system is responsible for breaking down the food into absorbable nutrients, where mechanical and chemical digestion takes place. During exercise, the muscular system is engaged as it enables the body to move and run. Finally, the urge to use the restroom indicates the excretory system is working to remove waste from the body. The urinary system forms a significant part of the excretory system, but so does the digestive system in the form of defecation.