using System;

using System.Text.RegularExpressions;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        string text = 

@"THE PEN IS ON THE TABLE

The cat, not the rat is UNder ThE Table (note comma)

The ChiLDreN Are ON The trampoline";

        string result = Regex.Replace(text,

                @"\p{L}+",

                match => match.Value.Length > 3

                    ? CultureInfo.CurrentCulture.TextInfo.ToTitleCase(match.Value)

                    : match.Value.ToUpper());

        Console.Write(result);

    }

}