A back to basics question please.

A table only ever has one record with three fields, dt1, dt2, dt3.
In a multi-user environment, users will update one of these fields.

Example:

User 1: changes dt1.
User 2: changes dt2.

Here is the problem: When user 2 saves the changes to dt2, the changes made by user 1 to dt1 are lost.

How do I ensure that all changes are accumulative. The last user to update a particular field (say dt3) should stick, but any changes made in the meantime by other users to dt1 or dt2 should not be overwritten when dt3 is saved. How can that be achieved?

Cyril