Why is a dc voltage of 0.3v is applied in order to check the closedloop1 AC response?
The 0.3 bias voltage sets the output current at 3 Amps.
Through the feedback that same voltage will appear across the 0.1 ohm current sense resistor.
The AC measurements could be performed at different output currents by changing that voltage. In particular the gm of the output FET will change with the output current and that will change the open loop gain and thus the AC loop characteristics.