Answer:  3 cents.  During the Civil War, when people were hoarding coins for their metallic value or melting them down to make ammunition, the U.S. Bureau of Engraving and Printing started issuing postage-stamp-size “fractional currency” in 3, 5, 10, 25, and 50 cent denominations.  The practice ended in 1876.

 

Source:  The 365 amazing trivia facts calendar for 2007