In a bid to possibly ensure that there is a difference between its regular iPad and iPad Pro lineup, Apple limited multitasking to two apps on the former. On the iPad Pro though, one can run two apps side-by-side, launch another app and interact with all three of them at the same time. With iOS 12, Apple is bringing the same functionality to the 5th and 6th gen. iPads.
On the public beta of the iOS 12, 5th and 6th gen. iPad owners are reporting that they are able to run two apps side-by-side and then launch another app and interact with all three of them without any issues. Previously, launching the third app would lead to the two apps in the background being frozen.
It has been unclear as to why Apple limited multitasking to two apps on its regular 5th and 6th gen iPads on iOS 11 and older versions of the OS. While both tablets feature a slower chipset compared to the iPad Pro, they come with the same amount of RAM (2GB) as the 9.7-inch iPad Pro clearly pointing to this limitation not being due to the lack of resources.
Nonetheless, whatever the case might be, Apple seems to have removed this restriction in iOS 12. The restriction has only been removed from the 2017 and 2018 iPad though, with iPad Air and Air 2 owners still unable to run and interact with three apps at the same time.
There has been no official word from Apple regarding the removal of this multitasking restriction. Since the company did not mention anything while unveiling iOS 12 at WWDC 2018 either, it is always possible that it reverts back to the older restriction with the final release of iOS 12 later this year.