First, highlight and copy this symbol:
Then go to Settings - General - Keyboards -Text Replacement
and click the + sign to add a new Text Replacement.
Into the "Phrase" field paste the ☰ symbol, and into the "Shortcut" field you need to put the word or characters you will type when you want the ☰ symbol.
In my case I used "333" as the shortcut, so if I type 333 then the replacement ☰ pops up instead.
It's taken ages to explain, but it's really simple! Have fun with your ☰!