using System;

using System.Collections.Generic;

using System.Linq;

​

class MyObject

{

    public string First { get; set; }

    public string Second { get; set; }

    public string Third { get; set; }

}

public class Program

{

    public static void Main()

    {

        List<MyObject> objects = new()

        {

            new() { First = "ABC", Second = "DEF", Third = "GHI" },

            new() { First = "JKL", Second = "MNO", Third = "PQR" },

            new() { First = "STU", Second = "VWX", Third = "YZ" },

        };

        var propertyStrings = objects

            .Select(obj => new[] { obj.First, obj.Second, obj.Third })

            .Select(properties => string.Join("_", properties));

        var objectString = string.Join("\n", propertyStrings);

        Console.WriteLine(objectString);

    }

}