I keep hearing this pitfall, and then I keep seeing the results of idiots who don't understand that by not picking three totally at random, what they have creat

I keep hearing this pitfall, and then I keep seeing the results of idiots who don't understand that by not picking three totally at random, what they have created is a giant mess should any of those three fail, instead of a quick and easy to clean up mass that you are constantly raid rebuilding from.