Board Thread:Support Requests - Designing Your Wiki/@comment-28905188-20170826111910/@comment-25024589-20170826112719

A third level of navigation can be added to MediaWiki:Wiki-navigation as follows (with modification to suit your wiki, of course): The first level of navigation is preceded by one asterisk, the second levels go below the first one and are preceded by two asterisks, and the entries in the third levels go below the second ones and are preceded by three asterisks.
 * Yo, this is level 1
 * And this is 2
 * And 3!
 * And maybe another 3

You may also refer to Help:Navigation, as GreatveemonOne posted.