Is a DUI, which was a misdemeaner, considered a criminal conviction? It was a first offense and there was no fine, prison, or license revoked. Attending class was required.

Yes, it is a criminal conviction, but it is also a misdemeanor and not a felony.

But in some states I understand a third DUI can be a felony

In the first time, you are treated as a misdemeanor, but after a repeated practices of DUI, it would be suppose as a criminal conviction or felony charges, as a result you might loose your driving license, heavy charges or in a long term jail. This rule is applied in almost every state of the America and some other countries as well.San Mateo DUI lawyer (http://www.defendca.com), is the great experience for me which direct me about what to do when you under the trap of DUI.