simple multiplication in GF(8)
Yes, you are correct.

The modulo operation (in general, not only over polynomials) is defined such that this is always true:

$A \equiv A + B\ \  (\bmod\  B)$

Thus, if $A = X^3$ and $B = X^3+X^2+1$, that implies that $X^3$ is equivalent to the sum of $X^3$ and $X^3+x^2+1$, which is $X^2+1$, which is represented by $101$.