Your ISP doesn't have 3 open resolvers. Your neighbors have.
Your ISP could terminate the contract with those 3 customers, but they won't - they're customers. They could block inbound trafic on port 25 to those 3 customers (if they knew about it), but they have no incentive to.
Or they could block all inbound traffic on port 25, which will likely break DNS for a lot of customers.
Your ISP could terminate the contract with those 3 customers, but they won't - they're customers. They could block inbound trafic on port 25 to those 3 customers (if they knew about it), but they have no incentive to. Or they could block all inbound traffic on port 25, which will likely break DNS for a lot of customers.