To solve the equation \( 3 = -\frac{p}{4} \), we can start by eliminating the fraction. We can do this by multiplying both sides of the equation by -4:
\[ -4 \cdot 3 = p \]
Calculating the left side:
\[ -12 = p \]
Thus, the value of \( p \) that is a solution to the equation is:
\[ \boxed{-12} \]