While multitasking with two apps on the screen, you can add a third floating app into the mix. This can be useful when using Apple Messages or an email application that you want to check quickly without exiting your apps that are already in split view.
To perform this function, begin by opening your primary app. Then, open a second app through multitasking, but leave the window in the center of the screen without locking it into place with the primary app. Swipe the floating app off the screen to the right, then set up your multitasked app with the primary app and a locked in second app on the screen. To view the floating third app, swipe from the far right of the screen to the center to view and interact with the third floating app. If you need a refresher on multitasking apps in general, here is a support article.