A single hyphen should never be substituted for a dash. The hyphen's purpose is to connect. The purpose of a dash is to separate.
In days of yore when we used machines called typewriters, typists created an em dash by typing two consecutive hyphens. If we do so now, modern software often wreaks havoc by converting the double hyphen into one hyphen-thereby connecting what we intended to separate—as with the nonsensical hyphen-thereby in this sentence.
In casual writing, if our platform does not allow us to use special characters of the em-dash sort, one workaround is to substitute three hyphens for the em dash. When our text is then converted to HTML or RTF, our dashes won't become hyphens.
(Extracted from EE4, Chap. 2, "Fundamentals of Citation." If your recollection of the differences between em dashes and en dashes, 2-em dashes and 3-em dashes has grown fuzzy, see particularly EE 2.62.)