These three signs occurred on the American Continent when the Savior was crucified (see 3 Nephi 10:13) but in this context, Nephi is referring to the last days. The images of blood and fire are commonly used in describing the destructions of the last days, but the image of a "vapor of smoke" is not. This particular sign was explained by Elder McConkie:
Bruce R. McConkie
"It may be, for instance, that nothing except the power of faith and the authority of the priesthood can save individuals and congregations from the atomic holocausts that surely shall be." (Conference Report, Apr. 1979, p.133 as taken from Latter-day Commentary on the Book of Mormon compiled by K. Douglas Bassett, p.75)