3 sandwich choices
2 side item choices
4 beverage choices
Let's say the sandwich choices were PB&J, Ham, and Turkey
Let's say the sides were fries and mandarin oranges
Let's say the beverages were water, orange juice, milk, soda
PB&J
Ham
Turkey
Fries
Mandarin Oranges
Water
OJ
Milk
Soda
The combinations can be as follows:
PB&J, Fries, Water
PB&J, Fries, OJ
PB&J, Fries, Milk
PB&J, Fries, Soda
PB&J, Mandarin Oranges, Water
PB&J, Mandarin Oranges, OJ
PB&J, Mandarin Oranges, Milk
PB&J, Mandarin Oranges, Soda
Ham, Fries, Water
Ham, Fries, OJ
Ham, Fries, Milk
Ham, Fries, Soda
Ham, Mandarin Oranges, Water
Ham, Mandarin Oranges, OJ
Ham, Mandarin Oranges, Milk
Ham, Mandarin Oranges, Soda
Turkey, Fries, Water
Turkey, Fries, OJ
Turkey, Fries, Milk
Turkey, Fries, Soda
Turkey, Mandarin Oranges, Water
Turkey, Mandarin Oranges, OJ
Turkey, Mandarin Oranges, Milk
Turkey, Mandarin Oranges, Soda
So your answer is correct: There's 24 lunch combinations that can be made.
Hope that helps!