How to raise a proxy and bind it to the selected network interface?

Question

proxy

Firefox copy #1 ==> Proxy server ==> PPP connection (3g modem #1)
Firefox copy #2 ==> Proxy server ==> PPP connection (3g modem #2)
Firefox copy #N ==> Proxy server ==> PPP connection (3g modem #N)
How to implement such a scheme?
That is, with the HELP OF WHAT, can you raise a local proxy and bind it to a specific network interface?
Preferably on Windows.
And also, in the future, there will be a program that will restart the connection (disconnect connect) on the interfaces.