You will be able to reach the subnets that are advertised between the MX devices.
the relationship is between the MX devices, not between Organizations or Networks.
It might be easier to consider it just a site to site VPN with a third party. Take out that it is a Meraki, and it might be easier to think about it.
I think what you will end up with is
Site to Site VPN from Org A -> Org B
Site to Site VPN from Org A -> Org C
Org B and C are not talking to each other. Unless you create a third Site to Site, Org B -> Org C