Maybe just add routes and adjust firewalls? Tell routers A and B that 192.168.30.0/24 is behind xxx.xxx.xxx.54, etc. Then allow this traffic in. That's if it's all safe and you don't mind this traffic going over the wires as is. Otherwise you might need some encrypted tunnels.
Sob already solve It, but to be more specific and not change you topology, you can accomplish by doing:
1) Add a static route in router A, with networks reachable by routers B and C:
/ip/route/add dst-address=192.168.20.0/24 gateway=xxx.xxx.xxx.53
/ip/route/add dst-address=192.168.30.0/24 gateway=xxx.xxx.xxx.54
2) Add a static route in router B, with routes reachable by routers A and C:
/ip/route/add dst-address=192.168.10.0/24 gateway=xxx.xxx.xxx.52
/ip/route/add dst-address=192.168.30.0/24 gateway=xxx.xxx.xxx.54
3)Add a static route in router C, with routes reachable by routers A and B:
/ip/route/add dst-address=192.168.10.0/24 gateway=xxx.xxx.xxx.52
/ip/route/add dst-address=192.168.20.0/24 gateway=xxx.xxx.xxx.53
4)Allow traffic in firewall for those networks, in case you have any rule blocking it. Remember to put it above drop rules. Something like this:
/ip/firewall/filter/add chain=forward dst-address=192.168.10.0/24 # Routers B and C
/ip/firewall/filter/add chain=forward dst-address=192.168.20.0/24 # Routers A and C
/ip/firewall/filter/add chain=forward dst-address=192.168.30.0/24 # Routers A and B
Also you should have a rule to allow stablished and related connections.
There are a many others ways to do It, even better ways. This is just the simplest one.