Honestly, "saving" your credit is probably going to be an impossibility. If your name is on the mortgage loan, and "he" is already 3 payments behind, then the credit bureaus will look at it like you too are 3 payments behind. If you can pull off a short sale, and the odds are stacked against you, then your credit will be damaged. If he lets the property be foreclosed, then your credit will be damaged. I'm sorry, but I can't personally see any way you can avoid being credit damaged -- and yet, I'm not an attorney or a CPA, and this is not legal advice. So as others have already said, you would be well advised to consult with a competent attorney.
On a side note, I personally think divorce judges should be hung out to dry for approving an arrangement like you got. With your name off the title to the house, you have absolutely no say-so whatsoever in the disposition of the property. And yet with your name on the mortgage loan, your credit will always be at risk, based on whatever your ex-husband does - with no say-so or power whatsoever. I'm just sorry you didn't have legal counsel before that happened.
Best of luck to you!