>>398
C++11でstd::vector使っていいなら、これで十分でしょう

#include <vector>

std::vector<double> func(double x){
double x1, x2, x3;
x1 = x * rand() / RAND_MAX;
x2 = x * rand() / RAND_MAX;
x3 = x1 * x2;

return {x1, x2, x3};
}

auto x_calc = func(x);