You must have adapted your Percussion map, since I cannot get the Fusion Drum Set to produce a Tambourine sound at all. Page 133 of the JABB 3 PDF manual says that MIDI #54, assigned to tambourine in GM kits produced a Ride Bell sound in Fusion Drum kits.

Perhaps your best bet is to create a non-playing staff for your drummer and then use hidden staves to produce the mock-up sounds you want.

I have not changed much of FInale's default settings. In my setup, the tambourine i assigned to Midi #54 (F# below middle C"). I have use a percussion to specify in which staff positions I want the different percussion instruments placed, but that should not affect playback.

"Perhaps your best bet is to create a non-playing staff for your drummer and then use hidden staves to produce the mock-up sounds you want."
Yes! if there is now way to lower only the sound of the tambourine, that is what I might need to do...

Okay, I found your tambourine. (Can't believe it is too loud, but that's up to you.)

The key is to put the tambourine in a separate instance of the Aria Player, which you can tone down with the slider in the Banks and Effects panel.

Can you describe a little more detailed how I can do this? Do I do it from inside Finale, or do I launch the Aria Player app? I haven't really worked with this before, and Google-search didn't really help me...

Set up a second instance of Aria in the Banks & Effects panel (by pulling down the menu in VST slot #2).

Then click the pencil beside the Bank 2 Aria Player and manually assign the Tambourine to a slot in that player.

Then in Score Manager set the MIDI bank and channel to point to the added Tambourine.

If you use a Percussion map that matches the Tambourine slot on the original drum staff to a non-paying sound, you can use the original drum part for printing and then draw the Tambourine sounds from the hidden active one-line Tambourine staff.

Besides having greater control over your VIs (virtual instruments), there's another advantage to using multiple instances of Players.

Unless it is designed to access multiple cores in a single instance (UVI and Kontakt are not), each instance of a player will access one core only. If you find your system overloading. you can create an instance for each instrument. This lets your OS balance the load among the number of cores you have. This happens under the hood—nothing else for you to. 

It's a DAW trick but applies here, too.

Adrian: Thank you very much for the good instructions. I will test it shortly... :)

Mike: Great tip. I'm currently scoring just for a 10 piece orchestra on a quite faste macbook, so I have no problems whatsoever. But nice to know when I score for Symphony Orchestras.

Some people put different sections (winds, strings, percussion, etc.) into different instances of Aria (or another VST) because it allows them to adjust reverb and other effects to handle volume and an impression of distance and spacial dispersion.