Oh, I get it..
Correct me if I'm wrong
Cov(X,Y) = Cov(X1+X3,X2+X3)
= Cov(X1,X2+X3) + Cov(X3,X2+X3)
= Cov(X1,X2) + Cov(X1,X3) + Cov(X3,X2) + Cov(X3,X3)
but because there's assumption that Xi independent, so
Cov(X1,X2) = 0
Cov(X1,X3) = 0
Cov(X3,X2) = 0...