Windows – How to enable multicast routing in Windows XP

If you are trying to get traffic, including ICMP pings, across a different subnet, then forwarding needs to be involved. or the interfaces need to be on a router. Other than enabling Internet Connection Sharing, which restricts the IP ranges handed out on a second interface, I don't know how to enable IP forwarding on Windows.

Your 3rd subnet sounds like it shares some of the same network bits as interface B, and that's why you can ping B.

The proper way to do what you are trying to do is change the IP of interface A to be in the same subnet as interface B, and make sure your 3rd subnet is also the same subnet.

Without BGP and 'business class' ISPs multihoming isn't going to work for you. It is possible to do 1-to-1 NAT with IPv6. I don't know how to configure it on Linux though. There is another way though:

You will get a prefix from both ISPs. Probably a /56 or a /48. It doesn't really matter in this case if the link to the ISP gets it's own addresses or not. IPv6 can work perfectly with unnumbered uplinks. What you then do is to connect a router from each ISP to your LAN, and both should advertise a /64 from their own ISP on your LAN. So all your systems get (at least) two public IPv6 addresses: one from each ISP. You can give the advertisements of your primary ISP a higher priority. When the link to the ISP goes down the corresponding router should withdraw its advertisement and your systems will start using the other link.

The only downside is when one ISP doesn't accept packets with a source address from the other ISP. If that is the case then the routers should check the source address and forward traffic with the wrong source address to the other router.

If you only get a single /64 from your ISP then you can use that on your LAN. They should give you more than that though. A /48 used to be the standard for many years, a /56 is becoming more normal these days, but you should definitely get more than a /64.