$$log_{3} \frac{81}{100} - log_{3} \frac{3}{100} = log_{3} (\frac{\frac{81}{100}}{\frac{3}{100}}) = log_{3} (\frac{81}{100} \cdot \frac{100}{3}) = log_{3} 27 = log_{3} 3^3 = 3$$
Ответ: 3