I would bank on a lot more Forniteplayers than 3M. It was downloaded more than 2 million times in the first 24 hours so just another million downloads in 4 months seems way too low.
Well since it's free, I doubt many waited to download, thus basically near everybody who had a Switch and wanted the game at the time downloaded it already back then, hence why I don't expect that much growth right now.
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454
The Steam/GOG key gifting thread: https://gamrconnect.vgchartz.com/thread/242024/the-steamgog-key-gifting-thread/1/