I’m starting with Rule 2 because I don’t like Rule 1.
2. In a series of three or more terms with a single conjunction, use a comma after each term except the last.
red, white, and blue
honest, energetic, but headstrong
He opened the letter, read it, and made a note of its contents.
This is also the usage of the Government Printing Office and of the Oxford University Press.
In the names of business firms the last comma is omitted, as
Brown, Shipley and Company
The abbreviation etc., even if only a single term comes before it, is always preceded by a comma.