I am uncertain what you want to do and when you want to do it.
If you have a table of existing data that you want to modify every record, you could make an update query. The portions of update query are as follows.
table: Your table
UpdateTo: The sum of the fields you want to add.
Criteria: (([field1]=[field2]) AND ([field2]=[field3]))
IF this is something you want to do on a form, then there must an event that will trigger the analysis and the result of setting field6 equal to the sum.
Clarify the problem and application if this wasn't helpful.
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