Oh that’ll get old when what3words takes off and everyone starts using the 3mx3m square around their front door as their password.
Know someone’s address? Rotate potential passwords from the site, in a spiral pattern around their address until you hit the correct three-word combination.
More seriously, though, (only slightly) if three random words is a good password, maybe the what3words guys are missing an opportunity for their database to be used that way. Pick a memorable place and there’s your password. All you need to remember is the memorable place (down to its 3mx3m square location).
(And it will never catch on / get critical mass because the US would never adopt it as nobody there knows what a metre is.) 
