You can't do that in the way you've shown. The routing to reach site C isn't shared across the AutoVPN, so Site A will never be able to communicate to Site C, and vice-versa.
The way you achieve it in a Meraki environment is by building a site-to-site VPN in Meraki which establishes endpoints in both Site A and Site B, and then in Site C you need to configure two VPNs, one which comes from Site A and one from Site B. The only other way is to introduce another VPN termination into the environment (e.g. a Cisco Firepower, or other firewall of choice) at Site B which terminates the VPN from Site C, and then you establish appropriate routing to/from the new VPN termination (firewall), and distribute this into the AutoVPN.