After three days of plum wine fermentation, add sugar. The fermentation time of plum wine takes about three days to one week, depending on factors such as air temperature and yeast activity. In the fermentation process, appropriate amount of white sugar can be added to increase the alcohol content and taste, but the following points should be noted:

1. Control the amount of sugar: too much sugar will lead to the sweetness of wine and affect the quality. Add 20-30 grams of white sugar to each liter of plum juice.

2. Wait for a suitable time: don't add sugar too early or too late in the fermentation process. It is more appropriate to add sugar after three days of fermentation, because yeast has higher activity and can better decompose sugar into alcohol.

3. Stir evenly: After adding sugar, stir the sugar evenly to ensure that each plum can be fully fermented.