Hi,
You can remove in the backend the variants which does not exists.
In the fornt-end, the drop down would always list all possibilities and if the variant does not exists, the product would not be available (like "out of stock").
If you want dynamic content in the drop downs, you have to create an override for the view and it would require some PHP skills.
At this moment, the best is to hide the other drop downs, in order to display just once. When the first received a value, it would display the second drop down and load his value depending the existing variants (which are all loaded in the page), etc.
It is not a very complex development but it requires some time.
Best regards,