If you have:
A = rand(3);
b = rand(3,1);
then the solution to the system can be simply computed as:
x = A\b
Or if you already have an LU decomposition of A, then:
[L,U] = lu(A);
xx = U\(L\b)
the mldivide
function is smart enough to detect that the matrix is triangular and chose an algorithm accordingly (forward/backward substitution)