I am learning this in Language Arts right now. I am in sixth grade, and we are writing 5 paragraph argumentative essays. I believe the counterargument should be in the 4th paragraph, and then you give more pros that will outnumber the cons, so the reader is convinced.
We usually use three prongs, to make our writing more organized. So if you do it this way, you can introduce your third reason in your fourth paragraph, and THEN provide a counterargument. At least, this is my opinion. :P Don't like it? Stop reading this. Go find something else! :) Have a nice day!