I would like to add three working days to a date time field.How should I do it

That's a tough one. Generally with a date/time field you would just add hours to it for the days but that doesn't work for business days. There might be a way to do this if you had a formula that determined the day of the week and if it was Friday it would add 120 hours which should result in it being placed on Wednesday of the next week, if that makes sense.

I've seen a formula for figuring out the day of the week on this forum before so it should just be a search away. There might be a better way to do it though.