using System;   

int num1, num2, num3, result;

Console.WriteLine("type in first number");

num1 = int.Parse(Console.ReadLine());

Console.WriteLine("type in second number");

num2 = int.Parse(Console.ReadLine());

Console.WriteLine("type in third number");

num3 = int.Parse(Console.ReadLine());

result = num1+num2+num3;

Console.WriteLine(result);

​