You probably don't need a cart.
First, you probably don't want to insert into field3. If field3 by definition is equal to field1*field2, then storing it at all is just an opportunity for an error. Just store field1 and field2 in the database and if you want to use all three use a query like:
SELECT *, (field1 * field2) AS field3 FROM table
and even though you only stored field1 and field2 you can always get the value of field3 in your results.