Hi,
If you have a route based VPN between A & B then you need to do the below steps:
1: Please check the proxy ID's on both the sides, whether it's none or 0.0.0.0/0 or any , shouldn't be any subnet specific otherwise you need have both the subnets configured properly.
2: On site A add a route for the subnet 10.10.x.x poiting to the tunnel interface. I hope Site B knows how to reach to subnet 10.10.x.x.
3: Site C should have routes for the subnet: 192.168.70.x via SIte B so traffic could go to the tunnel.
Thanks,
Vikas