Not necessarily. While Ulaanbaatar does have street numbers and such, they're not well-known and are basically never used when giving out directions/advertising. It's more like: The "yellow building to the right side of the big department store". Official addresses also rarely use street names, it's more like: Flat #5, Apartment #2, Microdistrict #4, Sukhbaatar District.

So in reality everyone just uses Post Office Boxes for mail, who often just call you and say mail's arrived and charge 400 tugrug.

That said, no-one I know has even heard about what3words, aside from those who read the initial press release. And I'm from Mongolia.