3. If that's the case means basically if that (what was previously said) is accurate, then...

The word comes from Latin "cadere" which means "to fall". From that origin it has branched out in numerous directions as a noun that conveys an idea of "that which falls" (or "befalls"= happens). In particular it can be a happening, a legal action or trial, a situation, a mishap, an instance or example (of anything), an opportunity, . All of these are "fallings" (things that happen). From the meaning of "legal action" it has acquired also the meaning of "an argument", especially a convincing argument. A legal trial, dispute, or contest is often called a "case". However, within that contest each party to the dispute presents its own "case", meaning its own best arguments. If you are accused of a crime, you find a lawyer to help you defend yourself by presenting your case. In your examples #1 and #2, "case" means "argument". In example #3, "case" means "situation".

Hi Guilherme. To make a case (for) means to argue or defend an idea. In number 1, your group believes there should be more women in the cabinet and you're trying to persuade others! In number 3, you're basically saying that if that is so/right/true etc, people shouldn't do it! 'If you miss a week's lessons, you will fail your exam' 'Oh..I didn't know this. Thanks for telling me. If that's the case, I will be careful not to do it (miss a week's lessons)