If the intervenor is a necessary party, the Defendants can attempt to remove the case to federal court on diversity jurisdiction. I imagine the court's inquiry will delve into whether the intervenor is indeed a necessary party to the dispute.
You'd better seek and retain top notch legal counsel on this. Don't try to fight this fight without an attorney.
Under 28 USC § 1446, "if the case stated by the initial pleading is not removable, a notice of removal may be filed within thirty days after receipt by the defendant, through service or otherwise, of a copy of an amended pleading, motion, order or other paper from which it may first be ascertained that the case is one which is or has become removable."
It seems that the defendant would be allowed to at least attempt removal based on the facts you provided (although the other parties can always argue for remand). This is really a question of fact and needs to be determined on a case-by-case basis. Without more information about your case, it's hard to give a definite answer.
This response is not legal advice and does not create an attorney/client relationship. This response is intended to provide general information about the matter in question. Often, the question does not include important facts that, if known, could significantly change the reply and make it unsuitable.