Saying that using random words is better than using a string of unrelated characters assumes that password crackers don't know how to combine words.

There are about 180000 words in English (but don't assume everybody knows 180000 words). That means that for three words, you have at most 5.810^15 options.

For a password with mixed case and numbers, you have 62 different characters. With, 9 characters, you have 1.310^16 options.

I can understand that people would prefer common words to remember them, but they are not that much stronger.

The best password is the password I don't need to remember: generate long random passwords, and store them in your browser, in Keepass, LastPass or whatever system of your choice.