Two ways to do it. Both assume you're using a form to enter the data. It's not really professional to have users enter data into a table directly. If that assumption is a problem, please post back.
1. Use a dropdown-box (combobox) for the field in table #3. Set it's property "Only list" to true.
2. Code a BeforeUpdate event procedure on the field using a dcount, for example, to check if the entered value is present in table #1 or #2.
Hope this helps.
I am setting up a database and I'd really like to make it as professional as possible..I have created two tables with Auto ID fields and a third table which uses a new auto ID and then allows the user to input an ID from the two other tables. I want a way of getting Access to notify you if one of the IDs you entered in the third table does NOT exist in the first two tables...any way of doing that?