The Buffalo Nickel is one of three nickel designs in a 20th Century type set collection.

It's a 20th Century type set collection—every denomination and design of coin the United States Mint has produced this century.  There are three nickels because that's how many different nickel designs have been minted since 1900—Liberty (1883–1912), Buffalo (1913–1938), and Jefferson (1938 to date).