Hi, and thanks for your question.
One cup of flour may weigh 4-6 ounces depending upon certain factors like sifting (always sift before weighing), packing it down into the cup, etc., so weighing your flour will be most accurate. 4.5 cups X's 3 equals 13.5 cups which, if measured at 6 ounces per cup, equals approximately 5 lbs. of flour. Convenient, since flour comes in 5 lb. bags. : )
Remember, you need to triple all other brownie recipe ingredients, as well, Your end product might come out best if you mix three separate batches (I realize you might be trying to avoid this) of the your original recipe and then combine them, to bake.
I hope your brownies come out great!
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