What is the square formula of adding three numbers in brackets? How to open it

What is the square formula of adding three numbers in brackets? How to open it

(x+y+z)^2
=[(x+y)+z]²
=(x+y)²+2(x+y)z+z²
=x²+2xy+y²+2xz+2yz+z²
=x²+y²+z²+2xy+2xz+2yz