I have a client needing to install supply and return lines for their Air-Exchanger/Heat-recovery unit. The house is 3 floors with cathedral roof. The two upper floors are "lofts" and open on both sides. To get an idea, make a right triangle 35' at the base. First floor is 1000sqft, second 600sqft, and third 200sqft, all centered. This is new construction so they are very flexible. The house is heated with radiant floors and convection wall heaters. They want to install the A/E in the insulated ceiling in the third floor. I need to install a supply coming from a basement intake (their decision, they want fresh air from a lower level) to the A/E on the third floor then supply the house back down all three floors along the corner of the cathedral ceiling. Returns will be on the opposite side of the supply. If I supply to 3 registers down along the ceiling, is it best to supply through 3 individual lines to avoid all the fresh air exiting the first register or will one duct be sufficient to get the air all the way down? Also, not too sure of the sizing for these.

First time encountering this scenario, any advice is appreciated.