In a series of three or more terms with a single conjunction, use a comma after each term except the last.
- One, two, and three little red jelly beans fell on the floor. (Yum)
- Coffee, tea, and milk were offered to us at the haunted house.
- She bought the present, wrapped it, and hid it behind the cat box.
When it comes to the names of business firms, the last comma is usually omitted. Follow the example of the establishment.
- Smith, Mercurio and Co.
Having fun with this? Information gathered from Elements of Style.