How can I get 3 NASs to talk?

I have a My Cloud EX2 which has been my NAS for the past 2 years.

I have expanded my network to include an EX2100 and EX2 Ultra.

The EX2100 is going to be my main storage unit, with the EX2 Ultra serving as a Backup and/or Archive. The original Ex2 is going to be used for storage for a side business.

What I want to be able to do is have all 3 drives be able to communicate with each other for easy file transfer, independent of the computers on the Network.

The My Cloud Dashboard and App only looks at one drive at a time, so far as I can see.

You may need a dedicated 3rd party application if would like to sync data simultaneously across all 3 devices.