Bivariate random variable

Question

Let x1,x2,x3. Be independent random variables with common pmf P[x=+-1]. Write x3=x1.x2. show that x1, x2,x3 are pairwise independent but not mutually independent

Answer 1

Hint:

What is $Pr(X_3=1)$? What is $Pr(X_3=1\mid X_1=1,X_2=1)$? Are these equal? What does this imply?