I'm trying to figure out a good way to be able to connect to all the cameras and download the photos (preferably as they are taken) from all the cameras in the session.
Here are two options:
Use SneakerNet. You can buy a pack of 10 16GB SD cards for less than $60. Give each photographer two or three cards. After they've been shooting for a short while, have them switch to new cards and hand you the ones they were using. You then download the images to your Mac using an SD reader, delete the images from the card, and hand the cards back to the photographers. By that time they'll have been shooting on their second cards for a little while, so have them swap out the cards and repeat the process. Downloading the images from the SD cards will be faster than doing it over WiFi.
Use Canon Image Gateway. The 6D doesn't have to act as a WiFi access point itself -- it can connect to your existing WiFi network. And it doesn't have to connect to your computer -- it can instead upload photos to online services via Canon Image Gateway. You could have all three cameras set up to upload photos to the same account, so that they'd all be collected in one place for you. There are a lot of drawbacks with CIG though -- the photos have to be fairly small JPEGs, and you have to select and send them one at a time. It works fine for sharing a handful of photos when you're away from your computer, but at least as implemented in the 6D it's not a great solution for sharing a lot.