Collection----<Trap----<Test (----< means one to many)
The most flexible approach for producing such a report is to base your layout on Test. With the layout based on Test, You can include fields from Trap and Collection on this layout and they should display the correct information from the Parent and Child tables on this Grandchild based layout.
One approach is to put fields from Collection into Header, Footer and Grand Summary layout parts. Put fields from Trap into sub summary layout parts when sorted by TrapID or some other field that uniquely identifies each trap record. Fields from Test would normally be put in the body layout part.
Then you use Go to Related Records, Perform Find and Constrain Found set as needed to pull up the Test records that you want and sort them by the "when sorted by" Field(s) specified in any sub summary layout parts to group your test records to make the sub summary layout parts visible.
Here's an old tutorial on summary reports that is still valid: Creating Filemaker Pro summary reports--Tutorial
Please note that this is not the only report option possible and the format of the Excel file that you need to produce via Export Records will put it's own set of constraints on what options work best for your report.