There is any difference in matlab between normal calculations to discrete calculations ?

When I am trying to compute x*x*x (when * means convolution) I dont get the right answer.

X[n]=(7^n)heviside[-n] and I know that x*x*x=0.5(n-1)(n-2)(7^n)(heaviside[-n])=y[n]

my code is

syms n; X=(7^n)heviside(-n); y=conv(x)*conv(x)*conv(x);