First of all, if he only got a 3 day sentence on a 2nd offense in 10 years DUI - he got a great deal assuming he was guilty.
Now, he can expect either to get a lot of sleep or a little sleep depending on the crowding situation and what day of the week he shows up to start his time on. Friday nights start to be the most crowded.
Unless Mr. Williamson has himself been incarcerated he doesn't know any more about what to expect than you do. I didn't answer your question the first time I saw it because I've never spent 3 days in jail either. The good news is that it will be over quickly. It will likely be highly regimented and boring, a little like the army. If he shows the guards respect they will treat him humanely. I doubt seriously that he has anything to fear from gang violence. Many people do have a fear of incarceration but that should go away after the first 24 hours or so especially since he knows he wont be there long.
In Los Angeles County, 3 days of jail is like being in a revolving door due to overcrowding. I don't know for sure in San Diego, but the experience is not something people look forward to.
You husband may be able to arrange to do his time in a private (city) jail -- through negotiation with the prosecutor if available -- which is a little more civilized, but (1) costs money and (2) means he will do the whole 3 days.