If the External router has three IP's of different Subnets, then you can Translate internal IP's on the PIX to anyone of those subnets. The PIX's outside interface only needs to be configured with one of the subnet addresses.
I have tried this and it has worked for me. Good Luck
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...