For some reason I have another pppoe connection aside from the default internet route, and obviously I've unchecked the "use default gateway" option in the advanced tab for the pppoe connection.
Now I want my LAN3 to use that pppoe connection, I've tried putting the IP I got from that connection, say 10.0.0.1 into the "IPv4 gateway" box, but the clients under LAN3 are still routed through my default gateway, i.e. the internet.