First time I hear of this "three words" - is this actually promoted? Canonical "correct horse battery staple" is 4. 5+ truly random should still be strong.
Perhaps because it is so simple: what matters for passwords is length. No other complexity metric (codeset, whatever) is even in the same race.
Personally, my passphrases are seven words or more, which gets me to over 30 characters.
"qwertyuiopasdfghjkl" or "aaaaaaaaaaaaaaaaaabc" are not stronger than "kmY7$®f0V".