Answer
The third derivative $ f'''(x)$ is given by $$ f'''(x)= -27 e^{12-3x}.$$
Work Step by Step
Recall that $(e^x)'=e^x$
Since we have $$ f(x)= e^{12-3x}$$ then the first derivative $ f'(x)$, using the chain rule, is given by $$ f'(x)= e^{12-3x}(12-3x)'=-3 e^{12-3x}.$$
The second derivative $ f''(x)$ is given by $$ f''(x)= -3 e^{12-3x}(12-3x)'=9 e^{12-3x}.$$ The third derivative $ f'''(x)$ is given by $$ f'''(x)= 9e^{12-3x}(12-3x)'=-27 e^{12-3x}.$$