%GigabitEthernet0/0: Warning: 2001:2::/64 is a Subnet Router Anycast
The way you've typed in the IPv6 address is the syntax for the anycast address: it's the
:: after the 2. What You may want instead is
R3(config-if)#ipv6 address 2001::2/64 This will make 2 the HOST ID. Whereas, the way you had it
:: is the host ID side...this is the anycast address of the router that is generated by the router. Think of it as being equivalent to the NetworkID address in IPv4.
If you want the syntax to remain as you had it you may want to add the EUI-64 after you address.