Hoi Johan @johanvb, I guess this is not possible with the default WordPress roles (where 1 Author role user can edit only their own channel and slides, but the other 2 Author users can not access that channel or slides).
The concept that you are looking for is grouping of users, where all users in a certain group have access to channels and slides that are created by any of the users of that group. I have no experience with this, but it sounds like something more WordPress users would need, so maybe there is a plugin for that. Also not sure if Foyer plays nice with plugins like that, but please give it a try!
Maybe ask the creators of this plugin if groups are possible with their plugin, or if they know how to do this:
https://www.ads-software.com/plugins/user-role-editor/
Let me know what you find!