The Silver Three-Cent coin has an unusual history. It was proposed in 1851 both as a result of a DECREASE in postage rates from five cents to three, and to answer the need for a small-denomination, easy-to-handle coin. The coins, featuring a shield in a six-sided star and a Roman numeral III, were physically the lightest weight coins ever minted by the U.S. Mint. The silver coins were known as “fish scales.” The term “trimes is often used today for these coins.
The silver three-cent coin was minted from 1851-1873 at the Philadelphia Mint. In 1851 only, the New Orleans Mint also struck some trimes. In later years there were very few minted, and the 1873 issue was minted in proof state only. However, an earlier date silver three-cent coin can be bought in circulated condition for a relatively low price. This unique coin (along with the silver dollar, the half dime, and the two-cent coin) was discontinued by the Coinage Act of 1873.
Source: Wikipedia (wikipedia.org)