First of all I think the killer is that the pix can only have one default route. Thus while your pix will receive traffic on any of 3 interfaces, the traffic back will take one and only one path, which probably won't work. You have to consider the path that inbound packets take from outside to inside servers and then back out the pix to the source.
You would be much better off implementing the multihomed routing on a separate platform, such as a 26xx or 3xxx series router. Additionally, you will have a much easier time if you get an ASN and do BGP, rather than having 3 separate IP address blocks.
IOS software has the following features that the PIX does not, that make multihoming without BGP possible: policy based routing, NAT route-maps, and multiple default routes
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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...