how to populate/add & read values to this map?.

typedef map<string, map<string,map<string,vector<int>>> > myNestedMap;

The combination Key1+key2+key3 is unique.

I'm guessing your map is from std::map. And I guess you are trying to iterate through it. Then, a suggestion to use a nested loop is below.

  for (map<string, map<string,map<string,vector<int>>>>::iterator it1=myNestedMap.begin(); it1!=myNestedMap.end(); ++it1) {
    // it1->first is the key
    // it1->second is the value
    for (map<string,map<string,vector<int>>>::iterator it2=it1->second.begin(); it2!=it1->second.end(); ++it2) {
      ...
    }
  }