You will add all the 3 pppoe lines along with the default route in the NetBalancer. You will enable NAT on them.
I am not sure what you mean to “change the default gateway to a balancer gateway”. All the PCs in your lan should use ZS as the default gateway obviously. ZS will automatically balance traffic among the configured gateways in NetBalancer.
Enabling RIP is a bad idea if don’t really need it. I don’t see why you would need it here.