Maybe let’s take a step back and understand why this needs to be done. Unlike other virtualization technologies, Qubes does not make it easy to share folders between Qubes. I believe this is for security reasons.
Connecting qubes this way will make it much easier for all of them to be compromised… maybe it would make sense to make these three qubes into one. But ultimately that’s your choice. Here’s a related discussion:
also, I’ve changed the title a bit. Feel free to change it back if you don’t think it applies well to your situation.