ok so first why are you stuck on the first steps when I'm stuck on an efficient way to compare data from 3 different students?
Ok here are some starters:
Have these libraries and the using namespace std

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

Next, declare your structure and stuff:

struct grades{
  string fname;
  string lname;
  int test;
  char conduct;
  string si;
} student[3];
int studentAverage(int t, char c, std::string s);
int studentCompare(vector<int>t);
int valueChar(char c);
int valueStr(std::string s);
void displayWinner();
void printGrades();
vector<int>order{};

I am only going to do studentAverage and studentCompare.

int studentAverage(int t, char c, string s){
  int cv;
  int sv;
  int total;
  switch(c){
    case 'A':
      cv=100;
      break;
    case 'B':
      cv=90;
      break;
    case 'C':
      cv=80;
      break;
    case 'D':
      cv=50;
      break;
    case 'F':
      cv=0;
      break;
    default:
      return -1;
  }
  if(s=="Outstanding"){
    sv=100;
  }
  else if(s=="Excellent"){
    sv=70;
  }
  else if(s=="Normal"){
    sv=50;
  }
  else if(s=="Low"){
    sv=40;
  }
  total=t+cv+sv;
  total/=3;
  return total;
}
int studentCompare(vector<int>t){
  if(t[0]>t[1]){
    return 1;
  }
  else if(t[0]<t[1]){
    return 2;
  }
  else{
    return 0;
  }
}

This is all I'm going to give you, I shouldn't be doing this in the first place. But you should be able to do the rest from here.

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