Just picked up the 3 XL and while I have historically used a magnetic mount in the car, reluctantly decided to forego it this time since the Pixel Stand came with it and I figured I may as well make use of that. The above notwithstanding, I reflexively went to put my 3 XL against the mount when I got into the car the first time, and was surprised that it stuck – in a (light) case – without a metal plate!
It isn't a very powerful grip compared to what a plate would offer, but I've been driving around town for the past few days and it has stayed perfectly in place (I keep it in landscape orientation - it seems less steady in portrait, but the former is my preference so that's no concern to me).
If your car has a pretty smooth ride/good suspension, and if the magnet in the mount is strong, it may work for you (note: try at your own risk!). The mount I use is
https://www.amazon.com/gp/product/B016GNYZIC/?tag=hawk-future-20&ascsubtag=UUacUvbUpU6420780 and it's a good bit stronger than the vent mag mount I'd used previously.