That's my iPhone's passcode. A single "3".
It wasn't hard to do. In "Settings>General>Passcode Lock", turn off "Simple Passcode". Now, type in a new passcode, but make it only numbers. A passcode made solely from numbers will still show the number keypad instead of a full keyboard.
Why "3"? Because it's just under "ok". Fast and easy on older phones.
"Hey! You're telling everybody your passcode! Isn't that dangerous?"