def fancy_concatenate(list_of_lists):
    concatenate_string = " "

    for my_list in list_of_lists:
        for things in my_list:
            if len(my_list)==3:
                concatenate_string+=things
    print(concatenate_string)


fancy_concatenate([["A", "B", "G"], ["C", "D"]]) #Example (the argument be a list containing lists again)