The addresses aren’t randomly generated, they’re permanently allocated their specific three word combination.
The English language is composed of 171,476 words, at least according to the Oxford English Dictionary. What3words removed any hyphenated words, curse words, and homophones. That left the team with 40,000 words, which composed some 64 trillion unique three-word combinations. Since the Earth has room for 57 trillion 3-meter squares on its surface, that left a few trillion extras lying around.
Source - Popular Mechanics
So unless you happen to randomly select an allocated combination of those 40 000 words, you’re probably going to get lost
Here’s a quick TED Talk from one of the founders where he explains the thinking behind W3W, with lots of reference to South Africa.