I can help you to write the update statement. But I would like to know why are you doing this? This will help you with 3 days but what happens with a whole month? or a whole year? Why do you have this table design?

You shouldn't store values that way, to show the results in a report would be ok (even if some people say it should be done in the front end).

Please, review the requirements and then confirm you need to do this.