Your other option (if you do not wish to simply use a power controller) is to get a much higher rated element. I am thinking of for example a 12kW 400v 3 phase, and run this on single phase instead.
Usually the 3 individual heater elements would be running in parallel, so each element might be a 4kW 240v single phase. Wiring them up in parallel on just a single phase reduces the current through each individual element.
You can get 3 phase elements in the same 2.25" threaded fitting up to 12kW 400v. You would have to calculate the equivalent power if you were to run one of these on a single phase instead.
As a very rough estimate 12kW 400v would pull 30A, suggesting the elements are 13.3 ohms in total. Switching that into 240v instead reduces the current to 18 Amps and the power to 4.3kW.
That is not accurate, as I have not factored in the series and parallel resistor networks complications, but you can see the general idea.
Oh I nearly forgot: You can get them here: http://www.milkingmachines.co.uk/uk2shop-11.htm