string str = "1,2,3;2,3,4;4,5,6";
var arr = str
    .Split(';')
    .Select(x => x.Split(','))
    .Select(x => new
    {
        first = x[0],
        second = x[1],
        third = x[2]
    })
    .ToList();

Enumerable.WhereSelectArrayIterator<string, <>f__AnonymousType0#10<string, string, string>> { { first = "1", second = "2", third = "3" }, { first = "2", second = "3", third = "4" }, { first = "4", second = "5", third = "6" } }

List<string>(3) { "1t2t3", "2t3t4", "4t5t6" }

arr.Select(x => x.first + "t" + x.second + "t" + x.third).ToList();

arr.Select(x => x.first, x.second, x.third).ToList();

public static IEnumerable<string> JoinByTab<T>(this IEnumerable<T> list) where T : class
{
    PropertyInfo[] properties = list.FirstOrDefault().GetType().
        GetProperties(BindingFlags.Public | BindingFlags.Instance);

    var listStr = list
        .Select(x => String.Join("t", properties.Select(y => y.GetValue(x, null).ToString())));

    return listStr;
}

public static IEnumerable<string> Join<T>(this IEnumerable<T> list,
    string separator,
    params Func<T, string>[] properties) where T : class
{
    foreach (var element in list)
    {
        yield return string.Join(separator, properties.Select(p => p(element)));
    }
}

var result = arr.Join("t",
        x => x.first,
        x => x.second,
        x => x.third)
    .ToList();