I have not really done many 3 gal batches so you will need to read the instructions that come with the system before use and don't forget to do an initial clean run with just some water to rinse everything then rinse again making sure to rinse the pump and impeller.
However after reading, what I have found is that in a 90 minute boil I lose approximately 1 notch on the center pole. The notches represent values of volume approximately. This is without the copper hood. Your experience may vary.