Step 1: The equation of a hyperbola with the center at the origin and horizontal transverse axis is given by:
\[
\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1.
\]
Step 2: For the given vertices, \( a = 3 \), and the semi-latus rectum gives \( b^2 = 12 \). Substituting these values into the equation, we get \( 4x^2 - 3y^2 - 36 = 0 \).
Final Answer:
\[
\boxed{4x^2 - 3y^2 - 36 = 0}
\]