To find the height of the firework at \( t = 3 \) seconds, we can plug \( t = 3 \) into the function \( h(t) \):
\[ h(t) = -16t^2 + 96t \]
Now substituting \( t = 3 \):
\[ h(3) = -16(3^2) + 96(3) \]
Calculating \( 3^2 \):
\[ 3^2 = 9 \]
Now substituting that back into the equation:
\[ h(3) = -16(9) + 96(3) \] \[ = -144 + 288 \]
Then, performing the final calculation:
\[ h(3) = 144 \]
Thus, the height of the firework at \( t = 3 \) seconds is \( \boxed{144} \) feet.