Consider f(x,y,z)=ax2+ay2+az2+2bxyzObserve that f(x,y,z)=f(y,x,z)
(x and y are interchanged)
f(x,y,z)=f(x,z,y)
(y and z are interchanged)f(x,y,z)=f(z,y,x)
(x and z are interchanged)
Thus f(x,y,z) is symmetric with respect to any two variables of x, y and z.