JFET stabilised wien bridge component ratios
Rf is a load on the opamp output. Its value cannot be less than 2k ohms for most opamps so maybe use 20k ohms. Then the resistance of the Jfet plus R3 must be 10k ohms for a gain of 3. Let R3 be maybe 4.7K then the Jfet can be 5.3k.

R4 discharges the capacitor and allows the Jfet to turn on when the output signal level is low. The capacitor value is selected to be a good filter at the lowest frequency you want low distortion.

Allow the output level to be high enough for the circuit to work well, then attenuate it with two series resistors to ground at the output.