I got a beta firmware from TP-LINK support and upgraded successfully. Still are checking whether can help. https://static.tp-link.com/2018/201803/20180330/TL-ER6120(UN)_v1_20180326_Beta.zip

Maybe you should configure IPsec from router a to b and b to c and c to a.

ER5120 V3 can setup the IP range for IP group.

I think it should be not able to do that.

Whotan wrote I have the same problem. Started 3 weeks ago aprox. I don't know whether it's a official firmware, but I try to upgrade the firmware and haven't see the problem yet.

If you mean that you need three sites to communicate with each other, you can set up two entries of LAN to LAN VPN on each router, then each router will establish VPN tunnel with other two routers.