Given that $P(x)$ and $Q(x)$ are polynomials of degree 3 with real coefficients,which one of the following is not true?

Question

$\begin{array}{1 1}deg\;[P(x)\times Q(x)]=6\\deg\;[P(x)+Q(x)]=3\\deg\;[P(x)-Q(x)]\leq 3\\deg\;[(P(x))^2(Q(x))^3]=15\end{array}$