@moulte answer is correct for normal software such as Chrome, Zoom, etc.
But, In my case, Windows 3D viewer is not normal software. It's a default one. Can't find the file location of the 3D viewer. (actually, we can find it but, If you use it, you just want to get administrator permissions for your Flutter windows.)
So, I found this method to do my task.
Created Function
void openFile(String filePath) {
Process.run('cmd', ['/c', 'start', '', filePath]);
}
Called it when user clicks the button,
onPressed: () {
String filePath = r'E:\Downloads\go.fbx';
openFile(filePath);
},
Now, It's on with 3D viewer.