I have an ancient BM 3.5 server that acts as a internet Gateway and web
proxy, its connected to a single broadband router with a public IP.

I would like to run the web proxy on a second broadband connection, so
that the main routing connection (site-site VPN and email) has more

Is there a way of running just the web proxy to a different connection ?

I put a third network card in the server and attached the broadband router
to it, but it seems to me that the web proxy automatically uses the
default route next hop for all its connections.

Any way of making (on a single server) the web proxy connect to a
different internet connection to the main routing one ?