Add values to a table based on values in another table

When a record is inserted or updated records in Table1 I want a record to be inserted into table3 for each record ID that is in table2 if the table2.id does not already exist in table3. What is the best way to do this? Could this be done with a trigger? If it could how would I write such a trigger. I have never written one before and this sounds like a hand full for my first effort. Your help will be greatly appreciated.

Thanks

You could do it in a trigger. They're really not hard. It's just SQL you want to run whenever a certain action happens on your table. Pretty straightforward. Another option is to use stored procedures to do all of your inserts and updates. Then, just code what could go in the trigger directly into your stored procedure.

Pete