SteamDB confirmed the success on its Twitter account earlier this day. On Steam's official statistics page, the record is shown at 3,106,358.
. @PUBATTLEGROUNDS has finally reached 3 million concurrent players! https://t.co/HIkZMh7mCB pic.twitter.com/eJwXXSfHJD
— Steam Database (@SteamDB) December 29, 2017