You said that your mother's husband was on the deed when they bought house #3. They then added you and your husband to the deed. So, unless the deed says otherwise, it would appear that you each owned 1/4 of the house. That means that, as far as capital gains tax is concerned, you each received 1/4 of the proceeds of the sale. You and your husband each have capital gain on your individual shares of the house. As Opus 17 pointed out, your taxable gain is determined by your ownership percentage, even if the money from the sale was not distributed according to those percentages. It doesn't matter how much money the closing attorney sent to each party. But if the money was not distributed according to the ownership percentages, someone might have made a gift to someone else, and might have to file a gift tax return.
Even if the four of you are responsible for equal shares of the sale proceeds, it's possible that you do not all have the same amount of taxable gain, because you may not all have the same basis. This is something else to discuss with your tax pro.
Hopefully your mother and her soon-to-be ex are also getting expert tax advice. Divorce attorneys often know less about tax law than they think they do. We often see questions from people whose divorce attorney put something in the divorce decree that is illegal or impossible under the tax law. Your mother and her husband also need tax professionals.
After I thought about this for a while, I realized that when you said QCD you might have meant quitclaim deed. But in the context of taxes it has a completely different meaning.
