At my arraignment I was offered a pretrial diversion with a not guilty plea. At the end of three months am I able to petition for factual innocence because I'd really like to get the arrest off my rap sheet.
You can, but it's VERY unlikely that it would be granted. The fact that you were offered pretrial diversion doesn't mean the DA is admitting your innocence and it doesn't mean you are admitting guilt. It's a compromise entered into in order to resolve the case favorably for both sides (usually more favorably for you). A finding of factual innocence is difficult to obtain when a case wasn't even filed. In your case, charges were filed, which makes it even more difficult.