You can do that with a routing/forwarding table in any OS [that provides access to routing/chains]. Forward+NAT the cable tether to the 3G network, but keep the

You can do that with a routing/forwarding table in any OS [that provides access to routing/chains]. Forward+NAT the cable tether to the 3G network, but keep the default gateway to WiFi, so the local applications will not use the 3G.