I don't think this lens will allow 2x 3x converters to it, it will make the f/stop very slow indeed. There are some that like the lens and others that don't. I would suggest you go look at the lens and try it out, that is the best way to decide on a lens.
This post is out of sequence as it is answering @lex's post. It wouldn't allow me to delete either so I could repost it. Stupid bloody system this new system is. And I say again, Photo-John the font size and look sucks big time.