R1

hostname R1
!
ipv6 unicast-routing
!
interface Ethernet1/2
no ip address
duplex full
ipv6 address FC00:12::1/32
ipv6 enable
!
ipv6 route ::/0 FC00:12::2

R2
hostname R2
!
ipv6 unicast-routing
!
interface Ethernet1/1
no ip address
duplex full
ipv6 address FC00:12::2/32
ipv6 enable
ipv6 nat
!
interface Ethernet1/3
ip address 192.168.23.2 255.255.255.0
duplex full
ipv6 nat
!
ipv6 nat v4v6 source 192.168.23.3 FC00:23::3
ipv6 nat v6v4 source FC00:12::1 192.168.12.1
ipv6 nat prefix FC00:23::/96

R3
hostname R3
!
interface Ethernet1/2
ip address 192.168.23.3 255.255.255.0
duplex full
!
ip route 0.0.0.0 0.0.0.0 192.168.23.2