Planning an amplifier using a transistor
Output impedance is specified by you as 3kΩ, this sets R4.
The ratio R4/R3 defines your gain in this circuit, which you specified as 3. You can calculate R3 now.
The parallel circuit \$R_1//R_2 = \dfrac{R_1×R_2}{R_1+R_2}\$ is specified by you as 15kΩ. The transistor's base will load the voltage divider, but this is easily less then 10% when the transistor's gain is 100 or more. You can neglect it.
All you need to decide on is the bias voltage at the base of the transistor. Start half way the supply voltage and see how it works. If you realize yourself that the voltage across R4 is three times higer than the voltage across R3, then you can calculate the optimum DC voltage for the base (which is 0.7V higher than the voltage on R3) for largest swing.


But this method of guesstimating is probably not what your teacher is looking for.