There are some people on the street who committed the same "recidivism" prior to the passage of the law. They are not in jail because all three crimes that they committed were prior to the passage of the Three-Strikes Law. Yet they committed the crime described.
The prohibition against ex-post-facto laws is to prevent the creation of a crime after the fact. To the extent that any person's crimes were committed prior to the enactment of Three-Strikes, the law is certainly after the fact.
I haven't followed the matter closely, but I believe that the US Supreme Court has recently ruled that sentence enhancements, such as those for using a gun in the commission of a crime, require that the facts substantiating the enhanced sentence must be submitted to a jury. If the same standard is established for the facts establishing recidivism, then we would have a situation in which the jury is considering facts regarding crimes committed prior to the enactment of the law against recidivism. How would that not be "ex post facto"?
It's not ex post facto because it's the will of the people.
/sarcasm
They are not in jail for 3 strikes, because they were not in violation AS OF WHEN the law was passed. If they commit a new felony and are convicted, they can then be charged with a NEW violation of 3 strikes IN ADDITION to whatever their new felony conviction carries. They are not being retried for previous crimes, but the fact of their previous convictions are evidence in the charge of violation of the 3 strikes statute.
I haven't followed the matter closely, but I believe that the US Supreme Court has recently ruled that sentence enhancements, such as those for using a gun in the commission of a crime, require that the facts substantiating the enhanced sentence must be submitted to a jury.
I'm not a lawyer, but my understanding is that 3 strikes is not an enhancement of a sentence for the third felony, but a separate violation. It can only be brought after conviction of the third strike, and not during the trial. Which is why the D.A has some measure of discretion on bringing the charge.