First, don't think in terms of character length, but in terms of number of words.
Fairly short (5-10 character) words, easy to remember, seemingly random to anyone other than yourself.
Ideally you want five to eight of them. Two or three for the first two keys and just one or maybe two for the 3rd key.
3rdKey.com will ask you for three characters from your 3rdKey, so a word that's easy to spell and easy to identify character positions.
These need to be random words that no-one would associate with you, no matter how well they know you, but they can have some meaning for you. e.g. Very few people ever tell anyone the name of their first crush, that person's name would not be associated with you, but would seem random.
Examples
Alternatively, use five or six entirely random words, Generate Random Words
Choose one of the words for your 3rdKey, the one where we'll ask for three characters within the word. Then split the other four to six words into two separate keys, they'll be your first and second keys.
Your keys must be easy to remember. They can not be recovered or reset later, nor can they be changed, so choose wisely.
But why use random words?
I have my five or six words, know what I'm going to use for each key, and am ready to register.
