Inserting lookup data in a field of a table

Not using forms or reports or data views here is what I need to do.

Lookup a value in a table based on two existing values of a record in the record to be updated.

Something like this:

Field1 - Field2 - Field3

Field1 is the result of a lookup based on Field2 and Field3. Can be Dlookup or SQL. User selects values for Field1 and Field2 and the result of the lookup is posted to Field1

I have not been able to figure out how to embed this in the definition of the table.

What database?

Oops MS Access. I assumed all questions here were about Access..sorry.

You could base the field1 value on a macro (if in Access) that takes fields 2 and 3 performs and manipulation on it and save that in field1.

Do you have to run the macro separately or will it auto run somehow.

You will need to set the fields attributes to call the macro. It has been a long time since I had to do any of this in Access.