I want to ask what is the dangerous of creating a seed phrase that generate 3 words only + passphrase (strong password 18 characters)? Is it safe? Is it possible to lose the funds this way? If yes, how?

The idea is, people just need to memorise 3 words + passphrase (of their choice) (strong password) rather than worrying that the (seed phrase 12 or 24 words) is stolen or lost. Any reason why this cannot be practical or recommended?

I understand that seed phrase can easily be guessed by a hacker but then he needs to be able to crack the 18 characters (combination of lower, upper case, number, special characters such as * ? >< !@#}|_-&^%$)