Actually I’m wrong - apologies.
With the balance routers in that configuration you have two options.
Option 1 use a routing protocol on the MPLS connected WANs so that each balance knows how ro route to the LANs of the other balance routers. In this approach you would use IP forwarding on the WAN then configure OSPF or BGP to advertise the routes to the other balance routers.
Option 2 use pepvpn to tunnel over the MPLS network from site B & C back to Site A, the WANs can stay in NAT mode. This way you can add internet connectivity later if you want to increase bandwidth / resilience. The balance 20 is limited to 30MBps of (encrypted) VPN throughput (60Mbps unencrypted).
Either would work. Option 1 will let you use max throughput of 150Mbps over the WAN, Option 2 lets you add resilience and failover to 4G (via dongles on SIte B and C).