consecutive_find() is a function that returns the beginning of a range that contains strictly N consecutive elements. It was part of a SO question that I answered.

consecutive_find.cpp

#include <iostream>
#include <vector>
#include <algorithm>

template <class Iter>
Iter consecutive_find(Iter first, Iter last, std::size_t n)
{
    Iter marker(first), lead(first);
    std::size_t count(1);

    while (lead != last)
    {
        lead = std::next(marker);
        while ((lead != last) && (*marker == *lead)) { ++count; ++lead; }
        if (count == n)
        {
            if ((lead == last) || !(*lead == *marker))
                return marker;
        }
        marker = lead;
        count = 1;
    }
    return marker;
}

void test1()
{
   std::vector<int> v{1, 2, 2, 2, 3};
   auto it = consecutive_find(v.begin(), v.end(), 3);
   for (int i = 0; i < 3 && it != v.end(); ++i, ++it )
   {
      std::cout << *it << " ";
   }
   std::cout << std::endl;
}

int main()
{
   test1();
}