Add an unstored calculation field to your table, defined as Get(RecordModificationCount).
After the import, the updated records will get a +1 in this field, while the added records will have the value = 0.
In addition to siplus' post, if your target table has auto enter creation and modification timestamp fields you could look at those to determine the 3 latest values in each. But you would need to do that soon after the import.
Thanks, that's exactly what I needed, works perfect!