Is it correct to replace dashes with hyphens and how should it be done?

I often see sentences like this–made when people don’t have the care or ability to input the correct character–where two hyphens are supposed to form an em-dash.

Based on the sizes of various dashes, I would assume you would represent them using hyphens as follows.

em-dash — : —

en-dash – : —

hyphen – : –

Is it correct to perform this substitution and are you really supposed to use two dashes rather than three in the case of the em-dash?