public network redirection over multiple links to a client

[Joe Nuts <joenuts@xxxxxxxxx>]

Hi guys, I have a question about routing a (piece of a) public network
to a client, over multiple internet connections.
The server in question receives traffic for both public network x, and
public network y, and has one connection to the internet. The client
has three connections to the internet. I have no problems setting up a
multipath route so that outbound traffic from the client is split
between the three network connections with the ip route add scope
global nexthop... .
I would like for the server to send traffic it receives destined for
pub network x across the three links down to the client pc. the ip
route add ... nexthop ... command only seems to work with multiple
interfaces, and the server only has one interface. (see diagram, also
attached)

            Client
              PC
           |-----|
           |     |
  LAN      |eth0 |
(Pub Net X) |     |
           |     |              |---|           |---|           Server
           |eth1 | --- Pub IP A | I |           |   |             PC
           |     | -------------| n |           | R |          |-----|
           |     |              | t | Pub Net X-| o |          |     |
           |eth2 | --- Pub IP B | e |           | u | Pub IP Z-|eth0 |
           |     | -------------| r |-----------| t |----------|     |
           |     |              | n |           | e |          |     |
           |eth3 | --- Pub IP C | e | Pub Net Y-| r |          |     |
           |     | -------------| t |           |   |          |     |
           |-----|              |   |           |   |          |     |
                                |---|           |---|          |-----|
                                                          (receives
traffic for Pub Net X
                                                            and sends
it to Client PC
                                                            over Pub IP A, B, C)

I'm hoping I can come up with some kind of kernel level solution, but
the fact that i'm sending traffic back out to the internet with a
destination address that would normally be routed back to me, makes me
think that i need to use some kind of tunnel.

If anyone has any ideas of what utilities i should use to accomplish
this, your direction is much apprecitated.

Thanks!
-Joe Comeaux

             Client
               PC
            |-----|
            |     |
   LAN      |eth0 |
(Pub Net X) |     |
            |     |              |---|           |---|           Server
            |eth1 | --- Pub IP A | I |           |   |             PC
            |     | -------------| n |           | R |          |-----|
            |     |              | t | Pub Net X-| o |          |     |
            |eth2 | --- Pub IP B | e |           | u | Pub IP Z-|eth0 |
            |     | -------------| r |-----------| t |----------|     |
            |     |              | n |           | e |          |     |
            |eth3 | --- Pub IP C | e | Pub Net Y-| r |          |     |
            |     | -------------| t |           |   |          |     |
            |-----|              |   |           |   |          |     |
                                 |---|           |---|          |-----|
                                                           (receives traffic for Pub Net X
                                                             and sends it to Client PC
                                                             over Pub IP A, B, C)