Three Dollar Indian Princess Head (1854-1889)
The $3 Gold piece is something of an oddity, wedged in size between the $2.5 and $5, exceedingly thin, weight 5.015 grams; 90% gold, 10% copper.

According to the Encyclopedia of US Gold Coins 1795-1933 by Garrett and Guth, "…the $3 gold coin's primary purpose was to facilitate the purchase of sheets of 100 postage stamps-in other words, it was the rich man's three-cent piece." The coin's production continued until 1889 when postal rates rose. The problem with this account, it does not appear to match historical postal rates. In any event, it's primary use today seems to be to complete a proper 12 coin gold type set.