That's not how it reads if I'm not mistaken. Of course it was written by lawyers and I ain't one. Seems you can field an emergency QB without adding to the 53 but he can only be activated by the first 2 exiting through disqualification or injury (ruled out) and not for poor performance. Teams that dress 3 on their 53 man roster don't have those caveats and the 3rd can enter under any circumstances, but are not allowed another QB with the emergency designation apart from the gameday 53.
I can't imagine any teams dressing 3 on their gameday roster and taking up a spot for an extra position player with this new rule.
According to Tom Pelissero
NFL owners approved the bylaw proposal allowing teams to dress a third QB without using an active roster spot, I’m told. A no-brainer to bring it back in the aftermath of the San Francisco playoff game