The amplitude of a sine function in the form \( y = A \sin(Bx + C) + D \) is given by the absolute value of the coefficient \( A \).
For the function \( y = -3 \sin x \), the coefficient \( A \) is \(-3\).
The amplitude is:
\[ \text{Amplitude} = |A| = |-3| = 3 \]
Thus, the amplitude of \( y = -3 \sin x \) is 3.