Unfortunately we do not have any experience with multiple HAT sound cards connected to one Pi. Since the Pi 3 is so affordable and at the same time versatile, we recommend using individual Pi’s for each sound card to ensure the optimal configuration and control for each. You might encounter issues with the recognition of multiple HAT cards on the same Pi.
Regarding the JustBoom hardware, they might be the solution you are looking for; their structure is built around combining multiple sound cards of theirs on one Pi. Here is a graphic showing the possible combinations of JustBoom sound cards: https://www.justboom.co/product-guide/