$$1/x=1/y-1/z$$
$x=-\frac{yz}{y-z}$
$z\neq 0\text{ and }y\neq 0\text{ and }y\neq z$
$y=\frac{xz}{x+z}$
$z\neq 0\text{ and }x\neq 0\text{ and }x\neq -z$