Yes, you are right, function BUSINESS_DAY_FORWARD should allow specifying the number of days to add. This is an oversight on our part and we will fix it shortly. It the meantime you can use the following workaround to add 3 business days:
MyObject.FollowUpDate = BUSINESS_DAY_FORWARD(BUSINESS_DAY_FORWARD(BUSINESS_DAY_FORWARD(MyObject.ContactDate)))