publish results to a third table.
do you mean *store the calculated value in the third table* ? well, normally
you wouldn't. a standard rule is that you don't store calculated values when
you're already storing the raw numbers; you just recalculate the values
whenever you need to display them (in a query, form, or report).
there are exceptions to every rule, of course, so if you have a valid need
to store a calculated value in a table: you can easily calculate a total in
a query. add both tables to the query, then create a calculated field with
the A + B expression (leave out the "= C" part). make it an Append query if
you need to add the calculated value to the table as a new record. make it
an Update query if you need to add the value to a field in an existing
record in the table.
hth