All mintage figures are reported in A Guide Book of United States Coins, 53rd edition, by R.S. Yeoman, ed. Kenneth Bressett (New York: St. Martin's Press, 1999).
Quantities reported cover the number of coins struck and have no reference to the quantity reaching actual circulation. Many issues were deposited in the treasury as backing for paper currency and were later melted.
The quantities reported by the U.S.Mint of $3 gold pieces from 1873 to 1877 are subject to doubt.