If the function \( f(x)=\dfrac{\sin 3x+\alpha x\sin x-\beta \cos 3x}{x^{3}},\, x\in \mathbb{R}, \) is continuous at \( x=0 \), then \( f(0) \) is equal to:
\( 2 \)
\( -2 \)
\( -4 \)
\( 4 \)
