I have a heater in a 3 phase (415V AC, 230V per phase) timber kiln drawing 43A per phase. How many kW is this?
Your calculation is correct. You have 30 kW of electrical heating.

For rough oil calculation you can work on a figure of 10 kWh heating per litre of oil. For 30 kW oil heating you will burn 3 L/hour assuming 100% of heat gets into the kiln. De-rate for flue losses. 85 to 90% is probably a good value for a condensing boiler.

But why do you want to burn non-renewable oil? Shouldn't you be burning renewable timber? Timber will give you about 4 to 4.5 kWh/kg and you're already in that business.