STL вам поможет
const std::vector<int>
first{ 1, 2, 3 }, second{ 4, 5, 6 }, third{ 7, 8, 9 };
std::vector<int> result=first;
std::copy(second.cbegin(), second.cend(), std::back_inserter(result));
std::copy(third.cbegin(), third.cend(), std::back_inserter(result));