I want to use 3-300 watt heater but most of the heat controller only have 2 outlet? what can I do? Thank
Based on the controller you choose, you need to make sure it can handle the total amps and watts I suppose? If so, you could add an adaptor/extension to connect two heaters to a receptacle, assuming it safe to do so with what you have.
Another option is just use two heaters that will come close to 900watts?