Is that criminal going to take the chance if there are 3 or more armed individuals in sight?
I don't know what goes on in the heads of criminals, but one thing I do know is that they use the element of surprise.
There is no amount of preparation you can do in order to counter that because, like you said, only the bad guy knows when they're going to strike.
What you want is a society where everyone is paranoid of each other.