[dektep]

My solution

$xyz(xy+yz+zx) \geq (xy+yz+zx)^2 \geq 3xyz(x+y+z) \therefore xy+yz+zx \geq 3(x+y+z)$ แต่ $xyz \geq xy+yz+zx \therefore xyz \geq 3(x+y+z)$