Different providers for different nat clients

[artem@artem.ru]

Hello!

I have a strange task and don't understand how to implement such scheme.

There is a router with 3 interfaces:

IF1: PROVIDER A
IF2: PROVIDER B
IF3: LAN

Clients served via NAT. There are about 15 clients.

Now, what i need to do:

By default all traffic from all clients goes to PROVIDER A via IF1.
But, if total incoming traffic for any particular client becomes over X 
Mb then that client
and only that client must be switch for PROVIDER B. The switch must be 
automatic and must
not use any software on the client side.
While i know how to count traffic i don't understand how to route 
external traffic to/from
nat clients on particular external interface.

Any idea how it is done?

Artem