var lines = new List<string>();
// read lines from file to lines list

var stringBuilder = new StringBuilder();

foreach(var line in lines)
{
    var semicolonCount = line.Count(x => x == ';');
    if(0 < semicolonCount < 3)
    {
        line = line.Replace(System.Environment.NewLine, string.Empty);
    }

    stringBuilder.Append(line);
}

var resultText = stringBuilder.ToString();
// write this into the file

string[] lines = theText.Split(new string[] { Environment.NewLine }, StringSplitOptions.None);
string[] resultLines = lines.Where(l => l.Contains(";;;")).ToArray();