It is an ongoing mystery to me why MC sees some Renderers, and not other, sometimes, and not others. I have never found a concise or reliable way to work out why or how to fix it. There is lots of discussion about Volumio and seeing DLNA Renderers of the forum though.
Not being on the same subnet, or even the same switch, is a common issue to investigate.
Having IP Addresses assigned by a different DHCP Server/Router to the Renderers, even if the subnet is the same, could be an issue.
Use one DHCP Server, and use Reserved IP Addresses for all your devices, Server and Renderers. That doesn't mean Static IP Addresses. It means using "IP Address Reservation" which might be called "Static Leases" in your router.
Trying a "Refresh Dynamic Zones" might fix the issue. Right-click Playing Now.
Start watching "Services & Plug-ins > Media Network" found in the left Navigation bar.
Filter down to "Device discovery server (SSDP)" and see if all of the Pi3's IP Addresses show up in the listing. They should.
If you filter to the MC Server you should see GET requests from the Pi3's as well, every couple of minutes.
Use AndrewFG's Digital Media Renderer Analyser (DMRA) and see if it can see all the Volumio instances on the Pi3's.
http://www.whitebear.ch/dmraIf the DMRA doesn't find any Renderers, check the "Use Windows Discovery" box in the top right corner and try again.
That should give you a starting point.