Public IP → xDSL router →
RB 1 10.10.0.10/24 NAT 10.10.0.10 external and internal port 8291
RB 2 10.10.0.11/24 NAT 10.10.0.10 external and internal port 8292
RB 3 10.10.0.12/24 NAT 10.10.0.10 external and internal port 8293
change default tcp port of winbox service to RB2 and RB3
what RK has posted will work on the mikrotik you are using as a gateway. It seems you are using the DSL as your gateway.
Your solution is very simple and does not require firewall rules. However… I would suggest you put your DSL modem into bridge mode and put one of your RB’s in the NAT postion if possible. You will have MUCH more control.