You don't have to wait 3 or 5 years.
Provided that you are finished any sentence requirement and are NOT under probation, you can safely apply. If you are still under probation, you can't apply, which it doesn't sound like you are if you have already gotten your record expunged.
As for expungement, you can mention it at your interview, but they don't care.
You must report this arrest and details on your application form.
You must include an original copy of the arrest record from the police.
You must include an original copy of the court documents, i.e. your conviction and conditions of your sentence.
It is NOT a crime of moral turpitude, however, your interviewer may choose to interpret it that way, not likely, but anything is possible.
Search this forum as there is a link to uscis regulation which stipulates a DUI arrest in itself is not considered a crime of moral turpitude and print a copy and bring that with you in case you get an interviewer that doesn't know their regulations.
The "5 year" thing is for serious issues that shows you've been a good person for 5 years and they consider you've reformed yourself if no arrests have happened since then, but this is not for DUI's as I previously stated.
How do I know? I had a DUI and applied for citizenship 1 week after my probation was shortened and dismissed by the court, approximately 15 months from my conviction, 20 months from my arrest. I received my citizenship no problem. At my interview, we did go over the court records to ensure I was eligible to apply, i.e. I wasn't under probation at the time of application.
So don't worry, apply and good luck, you'll get your citizenship, provided you have no other issues you didn't mention.