It may not be 16GB but the cost will be as if it was that much because your data amount would be prorated for those three days.

Say your contract was for 10GB for 30 days. That would allow you to only use a little less than a GB total in those 3 days before overages would have occurred.

Also, since you cancelled before the prior months bill was generated, they may have added the 3 days of use to the prior month.