Not sure what "class 1 ref" means, not all of us are from Ohio.
The only way to understand how to work 3 man is to work it. Reading lets you know what is where to stand, but working it is only the way to get a very good feel for it. If I were you I would go to any scrimmage I could and insist on working 3 man whether you get paid or not. At least you will not get to a real game completely cold.
Peace