In order to create the ≡ symbol in a menu (or even in headings or paragraphs) you may not be able to simply copy and paste.
In such an event you need to put some HTML entity code in as the menu text and allow WordPress to automatically convert it into symbol.
Found list here.
And just in case that site goes down I saved a pdf version here:
Just be aware that the original list has/had additional variations/info visible only when hovered over which are not visible in the pdf.