Unfortunately BSIM3 is not made to describe 600V power MOS devices. It is typically used for "logic" and "analog" devices with 3.3V or 5V supply and may be extended to somewhat higher voltage, but probably not 600V.
Did you check with the manufacturer of the device if they offer a spice model? Typically they do, and often it is a lumped element model, i.e. a subcircuit composed of several elements (MOS devices, JFETs, resistors, capacitors).
Or you may post here which device you are looking for and perhaps get a hint where to search for a model.