AFAIK, you can monitor 10 free/activated versions with the Pro license. So the 3 activated PCs can each monitor 10 PCs with the free version installed. So with GlassWire Pro you can potentially monitor 33 PCs.
Refer to the following How-To topic which I wrote for version 1, but AFAIK still applies to version 2, I included a couple of tables that indicates how many “moinitored/server” PCs can be monitored by the “monitor/client” PCs: