I know we have a few bio/chem people on here, hopefully somebody can help me out. I have an exam tomorrow, going over study guide questions and not sure about this one, my ochem is a little rusty

here is the question exactly as its written.

"Name the 3 major "R" groups of amino acids."

So if an amino acid has a generic structure of a central carbon with amino group, R group, and Hydrogen atom attached to it, Am I being asked what the most common(major) R groups are in the 20 common amino acids? or do the hydrogen, amino group and slide chain count as r groups since they are attached to a central carbon?

Also if the first guess was correct, what are the most common R groups in A.A's, Im studying the structures right now and I'm having a hard time finding a shared/common one

Thanks