How can i extract n elements out of p in a std::vector with X elements

I have a vector with X elements, how can I extract n elements (3 in the example below) out of p (6 in the example) from the vector? Here is how I did it.

vector<int> a;
a.push_back(10);  a.push_back(11);  a.push_back(12);  a.push_back(13);  a.push_back(14);  a.push_back(15);  a.push_back(16);  a.push_back(17);
a.push_back(18);  a.push_back(19);  a.push_back(20);  a.push_back(21);  a.push_back(22);  a.push_back(23);  a.push_back(24);  a.push_back(25);
a.push_back(26);  a.push_back(27); a.push_back(28);  a.push_back(29);   a.push_back(30);  a.push_back(31);  a.push_back(32);  a.push_back(33);
a.push_back(34);  a.push_back(35);

int X = a.size();

// code to extract 3 elements of 6 until the end of the vector
for (int i=0; i< a.size(); i+=6)
  {

        int j = i+10;

        if (i >= a.size())
        {
            break;
        }
        else
        {
            sub_indices.push_back(i);
        }

        if ( (i+1) >= a.size())
        {
            break;
        }
        else
        {
            sub_indices.push_back(i+1);
        }

        if ((i+2) >= a.size())
        {
            break;
        }
        else {
            sub_indices.push_back(i+2);
        }
  }
// display result would output 
10 11 12 (drop three elements) 16 17 18 (drop three elements) 22 23 24  (drop three elements) 28 29 30 (drop three elements) 34 35

I did it like this but can anyone tell me if there is a more efficient way?