The term 1860s Prez is a reference to Abraham Lincoln, the 16th President of the United States who served from 1861 to 1865. He was elected in 1860 and is often referred to as the 'Great Emancipator' for his role in abolishing slavery in the United States. He is also remembered for his leadership during the American Civil War and his famous Gettysburg Address. The three-letter answer to the crossword puzzle is ABE, which is an abbreviation of Abraham Lincoln's first name.