ABaytin

private IEnumerable<string> TakeByThree(string input)
{
    var remains = input;
    while (!string.IsNullOrEmpty(remains) && remains.Length >= 3)
    {
        yield return remains.Substring(0, 3);
        remains = remains.Substring(3);
    }
}

public string Contract(string input) =>
    new string(TakeByThree(input).Select(t => t[0] == t[1] || t[0] == t[2] ? t[0] : t[1]).ToArray());