IMAS 2019–2020 Junior Division Round 2 Question 3

Question 3

If each of the variables $x$, $y$, and $z$ is increased by 1 at the same time, then which one of the following algebraic expressions will remain the same value?

$x^2+y^2+z^2$
$x^3+y^3+z^3-3xyz$
$xy^2+yz^2+zx^2$
$2x^2-2xy+2y^2-xz-yz+z^2$
$3x^2-4xy+3y^2-2xz-2yz+2z^2$