Actually, while you were sleeping (hehe for timezones 
) I found another article that looks very promising:
But as I expected, you will need L3 functionality for this to work, i.e. a actual router where you can manage the routes in every site.
You would keep the 2 or 3 different subnets, one for each site (e.g. 192.168.1.x 192.168.2.x and 192.168.3.x) and then have a distinct overlay network on top of those subnets.
It MUST be different from the 3 physical subnets. For instance a 10.x.x.x network would be good as it would clearly show what is local and what is zerotier.
The 3 routers need to have zerotier running on a separate virtual interface. You should then setup the routing table on each router to find the other 2 192.168.x.x via the 10.x.x.x IP of the corresponding site’s router.
This being said, you will need 3 slightli intelligent routers for this to work. The gl.inet might work but could depend on model.
I now think it is possible, given the right hardware. Not sure if it’s clear to you, I can try to fill in the blanks if more detail is needed, so let me know if you need more help…
G.