Thanks to the data we now have in 2015, the 1878 is the most common date in the entire series of Three Dollar Gold pieces, and it is the most common by a wide margin over the 1854 and 1874. PCGS alone has certified thousands of this date in Mint State from MS62 to MS64. In MS65 and better, the populations drop off dramatically until MS67, where 1878 becomes a true-condition rarity. The finest example is a single PCGS MS67+.
The 1878 is known for its frosty luster which, on high grade pieces, has a shimmering effect that is simply lovely.. The strike is usually good, but sometimes weak on the knot at the base of the wreath. This date has been heavily counterfeited, so beware of raw examples offered in the marketplace.