seasonality correction

[Dragoneo]

I've got seasonality in the third quarter in a time series, and i'm doing an arima forecast. To correct seasonality via seasonal differencing i do: seascorrect=log(gold)-log(gold(-3)) or seascorrect=log(gold)-log(gold(-4)). I tried both cases and both end seasonality, but i don't expect both to be correct. Can you help me please?