<p>I can understand the use for one level of namespaces. But 3 levels of namespaces. Looks insane. Is there any practical use for that? Or is it just a misconception?</p>

Big codebases will need it.  Look at boost for an example.  I don't think anyone would call the boost code 'insane'.
If you consider the fact that at any one level of a hierarchy, people can only comprehend somewhere very roughly on the order of 10 items, then two levels only gives you 100 maximum.  A sufficiently big project is going to need more, so can easily end up 3 levels deep.