So, you feel felons should get three more whacks at violent crimes before we remove them from society for life. How about if any felony conviction carried a manditory parole provision requirement of life imprisonment if convicted of another felony? Is that enough fair warning for you?
How would you like it if your state passed a three-strikes law for moving violations in an automobile? On the third ticket you lose your driving privileges for life.
I would feel forewarned is forearmed and strive to not exceed my three. If I already had three when the law was passed, I would know I had no more chances left and drive accordingly. If I felt the law was too harsh or too widely applicable, I would lobby for its repeal.
Our Founders didn't want the government changing the rules after the fact. This was an abused practice of the monarchy that was overthrown. Most of the career criminals have records as long as your arm. Having three strikes against them is not really the problem.
Another thread describes a new gun control law being proposed which includes a "stockpiling" provision which makes it unlawful to build an inventory of arms prior to a regulation becoming effective but after it has been "prescribed". The government wants to punish a person for not pre-obeying a proposed regulation.