Assuming you managed to read to all appropriate documentation on basic MF and (date)functions........
Options:
- Set a validation rule on the field
- Create MF on the on-change event of the field
- Create MF for a custom Save event on storing the record
- Set that field with a default value of current-date + 3days (which works if most of the times it WILL be just that and for the fact that most users are going to be VERY lazy)
Actual incorporation of the logic will be dependant on your users.