The 1866 Three-Cent Nickel is one of the most common dates in the series, but it is much less common than the 1865 and several of the dates in the 1880's. Nonetheless, collectors should have an easy time locating Mint State examples, where MS-63 and MS-64 seem to be the most common Mint State grades. The finest examples certified by PCGS include a pair of MS-67s.
Clashed dies are the norm for this date, ranging from light clashing to heavy clashing and worn dies. The present writer cannot recall ever seeing an unclashed 1866 Three-Cent Nickel.