Please describe the purpose of each of your three tables and the relationships that link them. Without that information we can't really suggest much for how to set up your layouts to work the way that you need them to.
I can see FM allows for data validation but this seems to apply only when the record is created.
Not quite. Each time you edit a record and commit the data--which often happens automatically, the validations will be applied and any errors will trip a validation error message. Some data validation will be applied when the user exits a field as well--such as entering an invalid date into a field of type date.
You also have the option, if you have FileMaker 11 or newer, to use the OnObjectValidation tab to run your own script to check for and deal with errors before any built in or developer defined validation rules are used and this can produce a much more user friendly experience for your users.