This simple design was created using the obverse of the half dime combined with a simple reverse designed by William DuBois as an answer to a request by the Chairman of the Committee of Ways and Means, Sam Vinton in January 1849 for the creation of a 3 cent piece.
These were supposedly struck in an alloy of 50% silver and 50% copper and weigh 22 grains. To my knowledge, none of these have ever been analyzed. Restrikes were apparently made, presumably in normal 90% silver alloy and should weigh different. Supposedly obverse dies V-4 (per Judd/Breen unconfirmed) and V-8 (per Clint Cummins) were used to strike these.
Part of a February 2, 1849 letter from W.E. Dubois to Matthew Stickney in the Peabody Essex Museum related to this pattern is shown below courtesy of David Stone.
"I send a sample of a three cent piece, half silver and half copper; I do not believe it will be adopted, but if it is the dies will be different. There was another alloy of 60 percent silver, with III on the reverse; I could not spare you both as only 12 or 15 of each were struck."
This appears to be accurate as only about a dozen are believed to exist.
Photo courtesy of Superior.