Is time served for previous DWI offense considered or does it not count?

Time served for previous offenses is likely to be considered--but not in a way that is going to be helpful to your husband. If anything, it's likely to be a negative. Previous offenses are previous offenses, even if it's been twenty years since your husband got his last offense. I haven't reviewed the statutes in Texas, but generally speaking it's unlikely that they are written in such a way that your husband will receive any significant credit for good behavior during the last 20 years. All that's going to matter is that this is his third DWI, but it all depends on the judge and the specific wording of the governing statute. Sometimes the judge's hands are tied by specific language in the statute that mandates certain sentencing guidelines for repeat offenders. Sometimes the judge has more flexibility to take into account things like decades of good behavior. The other thing to keep in mind is that it's not necessary a positive that your husband previously served time for DWI offenses. It depends on what sentencing alternatives were available at the time. If there were diversion options (community service for example), it begs the question why a judge way back in the day decided your husband should do time instead of pick up trash along the highway.