What are the three rules of expo facto law?

Ex post facto laws are prohibited by the U.S. Constitution and refer to laws that retroactively change the legal consequences of actions that were committed before the enactment of the law. The three main rules concerning ex post facto laws are: they cannot criminalize actions that were legal at the time they were committed; they cannot increase the penalties for an offense after it has been committed; and they cannot change the rules of evidence to make conviction easier after the fact. These rules are designed to protect individuals from unfair legal repercussions and ensure a stable legal system.