2021 IIMC keystage3-IWYMIC individual Question 3

Let $P(x)$ and $Q(x)$ be two quadratic polynomials with integer coefficients and their leading coefficients are both $1$. It is known that $P(Q(0))=Q(P(0))=1$ and $P(0)+Q(0)=2$. Find the value of $P(3)+Q(3)$.