I have used DEV-C++ compiler for debugging purpose. But you can use any C programming language compiler as per your availability.

#include<iostream>
using namespace std;

int main()
{
    int num1, num2, num3;
    cout << "Enter three numbers: "<<endl;

    cin >> num1 >> num2 >> num3;
    if(num1 >= num2)
    {
        if(num1 >= num3)
            cout <<num1 << " is the largest number";
        else
             cout <<num3 << " is the largest number";
    }

    else if(num2 >= num3)
        cout <<num2 << " is the largest number";

    else
        cout <<num3 << " is the largest number";

    return 0;
}

Result:

Enter three numbers: 

80

90

60

90 is the largest number