Our boy is also just past 3 and a half and has too nailed the day time potty training. We just followed a basic outline in a book we found and it seemed to work and explained it in a way we could understand and get onboard with.
However we haven't tried nightsI think we are putting it off as we have a newborn on the way (due date next week!) And worry it may just be too much for us to juggle, but it's probably not that bad. I am worried we may be leaving it too late but he still has a wet nappy when he wakes and has taken to pooping in the nappy before we get up!! Or maybe while hes asleep, hard to tell.
In my experience, the overnight wet nappies are because they are relieving themselves when they first wake up in the morning, not because of overnight accidents.
You'll have a tough first few days until they break the habit (so make sure waterproof sheets are down), but both of my boys are now in the habit of going straight to the bathroom when they wake up in the morning (youngest is 4, but he's been dry overnight for ~ 12 months).
Limit their fluid intake for a couple of hours before bedtime, make sure they use the toilet immediately before going to bed and immediately when they wake in the morning, and you'll be 99% of the way there
