Your home builder will explain to you that, normally, all conventional homes with a basement or attic are already what they call 3-zone residences, because they have three floors or levels.
The traditional two-deck multi-zone systems in the average home are, frankly, very inefficient. Here's why: Two-deck systems use a combination of of heated and cooled air to reach the temperature needed. The constant volume of airflow, both heated and cooled, requires ongoing fan work, which uses a lot more energy than other systems. Because the traditional two-deck zone controls have the heating and cooling decks active at the same time, they certainly provide excellent zone temperature control. They also have a very easy design and operation, but create a situation where simultaneous heating and cooling occur.
"By adding a third deck, where the 3rd deck puts out recycled air (also referred to as "bypass air." www.jrwortman.com), mixed with fresh ventilation air, the decks are arranged in a manner that when the zone is calling for heating, bypass air and hot deck air are mixed to meet the zone requirements." (www.jrwortman.com)
Denver home builders recommend homes larger than 2000 square have more than 3 zones. In this way, if you spend little time in one or another zone, you do not have to heat them.