Expanding access to a site to site VPN tunnel

I have a (probably a bit silly) networking question regarding linking networks via VPN tunnels.

I have three sites, A, B and C

A can see B and B can see C, what are my options if I want A to access C?

If I want to access network C from A I can add network C to link 1 and I assume you have to add network A to Link 2 so the traffic gets back?

Is there any way of just using static routes instead of editing the encryption domains to get the same result?

https://i.ibb.co/QcQ7fW5/Capture.png

This problem comes about due to having a site to site connection to an external network that I can't change. But I want more than one subnet to access this external network.