What is the ruling on a husband saying to his wife: "I divorce you by three if you do not divide the company between you and so-and-so, you are unlawful to me," and does it take effect if the wife and the partner divide some of the company's assets but not all of them?

If the company was not entirely divided between your wife and the woman, then you have broken your oath, because fulfilling an oath entails doing everything that was sworn upon. As for your statement, "You are unlawful to me," scholars have differed regarding it. Some consider it ẓihār, some ṭalāq (divorce), and some an oath. What is held as fatwa with us is that if you did not intend anything specific, it is an oath. If you intended ṭalāq, it has occurred, and you may take your wife back if it was not the third divorce. If you intended ẓihār, it has occurred, and you must pay the expiation for ẓihār. If you intended an oath, or did not intend anything specific, then you must pay the expiation for an oath. One should avoid swearing by ṭalāq and by declaring something unlawful, for the prescribed oath is by Allah Almighty.