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How can I update one form field from another?

I have three tables that are related via Pet ID>Pet ID and Pet Name>Pet Name. I have a form based on a query that has all the info of each table in it. The form is tabbed and each tab is one table. How can I get the Pet Name field in the Additional Info Table tab and the Pet ID in the Behavior table tab to automatically fill in what is entered in the Dogs Table tab fields? I can't get the relationships or whichever to work out otherwise, because it won't save my record and shows errors.

placeholder text for bug in Chrome