rtadv設定がおかしいな
こうでしょ
ipv6 prefix 1 dhcp-prefix@lan3:1::/64
ipv6 prefix 2 dhcp-prefix@lan3:2::/64
ipv6 prefix 3 dhcp-prefix@lan3:3::/64
ipv6 vlan1 rtadv send 1 o_flag=on m_flag=off
ipv6 vlan2 rtadv send 2 o_flag=on m_flag=off
ipv6 vlan3 rtadv send 3 o_flag=on m_flag=off