My husband doesn’t have a middle name, but in his country, Mexico, he had two last names, his dad’s and then his mom’s. That’s typical in many Latin American countries. When he was doing his US citizenship paperwork I told him he could make his mom’s last name his middle name if he wanted to keep it, but he chose not to so in the US he has no middle name. I guess in Mexico he doesn’t have a middle name either, but he has three names.
Many American women move their maiden name to their middle name when they get married. I didn’t do that, because my mom wanted me to be named Leslie, but she didn’t win, so she made it my middle name, and when I got married I kept it in the middle. Sometimes I wish I had kept my maiden name, but it’s not a big deal either way.
My mom has a middle name, because the tradition is to name after a dead relative, or the first initial of the dead relative, and that was accomplished by her middle name. I guess my grandparents really like her first name. My mom’s sister has no middle name.
Middle names can give opportunity for a different name than the father’s name. John junior might be called by his middle name Eric.
I think using a middle name evolved for different reasons. You certainly don’t have to use your middle name. You can easily drop it on legal documents.
Back hundreds maybe thousands of years ago Jewish people were son of or Ben. Isaac Ben Solomon. Isaac son of Solomon. Maybe that started a trend for three names?