Independence lets a compound conditioning event drop out

Let $X_1,X_2,X_3$ be independent identically distributed random variables with probability mass function: $P[X=k]=0.2,0.3,0.1,0.4$ for $k=-1,0,1,2$. Then $P[X_1+X_2=0\mid X_2=1,X_3=2]$ is: