Unlike previous versions of iPadOS, you cannot disable multitasking features of iPadOS 15.x.
The three dots “button”, and the multi-tasking controls of which you complain, are a core element of the iPadOS15 User Interface - intended to simplify use of full/split/slide-over multitasking. This control cannot be hidden or suppressed.
Some Apps will require update, by their respective developers, to accommodate changes made within iPadOS. Until affected Apps are updated, there is little that you can do other than to seek assistance from the respective App Developer:
How to contact an app developer - Apple Support
Apple do invite constructive feedback, comments and feature requests via their product feedback pages. For iPad and iPadOS:
Feedback - iPad - Apple