Good idea mentioning FM6. The fact that you are using such an old version of FileMaker can limit some options available in the later versions.
You have this structure between your files(tables):
That shows one to many at each relationship and that makes the following approach work:
For reports that list records from all three tables, create them in the TrophyBreakdown file. If you define a relationship in TrophyBreakdown that matches back to InvoiceDetail and a relatinship in InvoiceDetail that matches back to Invoices, you can list the tropy parts fields in the body of your report layout, but include fields from the other two related files (tables) in either the header or a subsummary part. Use sub summary parts to strategically group your trophy part records listed in the body.