Civil War historian Gordon Kwock credits "Research work of Shaie Mei Deng Temple from New Orleans, Louisiana" for this wonderful item of information:
Pang Charley, Pvt. Co. G. 1st La Infantry En. ___ Federal Rolls of Prisoners of war. Captured near Chickamauga, Ga., Sept. 20, 1863. Forward to Military prison, Louisville, Ky., from Nashville, Tenn., Sept. 30, 1863. Transferred to Camp Douglas, Chicago,Ill. [in the city, about a mile south of the modern South Side Chinatown], Oct. 2 1863. Note: Co. G was named Orleans Light Guards Company.
[The National Parks Service's Civil War Soldiers website confirms that Company G of the 1st Louisiana Infantry Regiment did include a soldier named Charley Pang]
Kwock may have turned up the third Chinese in Chicago as well. He quotes another historian, Dr. Qingsong Zhang, as follows:
"Tsui Kuo Ying [崔國因], Chinese ambassador to the United States, wrote in his diary in 1891 that there was a Chinese named Ah Mei in Chicago. Ah Mei studied in a military school for several years and served in the U.S. Navy during the Civil War. After the war, he became an American citizen and was permitted to vote in important elections. The name and story has not been confirmed. However, it has been known that in 1865 there was a Chinese in Chicago who sent letters to relatives in China. Census also show that there was a Chinese in Chicago in 1870."
We do not know what is the source of Tsui Kuo Ying's statement "it has been known ..." and, in fact, we have not even seen the census record about the lone Chinese Chicagoan in 1870. But an 1865 date is plausible. By then, railroad travel from the East Coast, where there were already a good many Chinese, was easy, and there was much population movement in the U.S. at the end of the Civil War.
Reference: Qingsong Zhang, article written for Jeanne T. Heidler and David S. Heidler's Encyclopedia of the American Civil War, ABC/CLIO, 2000.
Research & writing by Ben Bronson and Chuimei Ho; copyright 2004-2006 by the Chinatown Museum Foundation.