Just type 3 dashes — and hit ‘Enter’ immediately after.
The other options are: Three asterisks***, three underscores___, three equal signs ===, three pound signs ###, three hyphens ---, three tildes ~~~
The types of lines differ based on the entries:
The tildes (~~~) gives:
Three underscores (___) gives:
Three asterisks (***):
Three pound signs (###):
And so on.
If you create one of these by accident, simply hitting the backspace will undo the line creation and leave you with three of the symbols you created.