My son is three and has recently been potted trained as well. What worked for us, once he had switched to underwear, was to reward him. He had a chart with the days of the week and his favorite cartoon/Sesame street characters. He loved putting the sticker on the chart and enjoyed seeing how many stickers he could get in one day. We also have a reward system in our home in which he collects rocks in a jar. Rocks are earned for good behavior, eating well, being respectful, etc. We decided that on days he had no accidents he earned three rocks. When the jar is filled, he receives a reward of his choice (typically a small toy, or an ice cream from the ice cream parlor, or a trip to the park).
In the first week, he had two accidents. In the following three weeks, he has had no accidents. He has already earned one reward which he loves.
Good luck.