Presumably you have updated to iPadOS 26. iPadOS multitasking has received a significant makeover from that to which you may be accustomed in iPadOS 18.x and earlier - and may take a little time to learn and adjust your workflows.
The three dot button to control multitasking is still present - but is visually now a traffic-light button. The button location has changed. Instead of being top-centre of the App window (a location which frequently conflicted with controls for many third-party Apps - of which regular complaint has seen here), the button is now at top-left of App windows that are not being viewed in full screen mode.
For Apps that are being viewed in full screen mode, a simple downward swipe from the top-edge of the screen reveals the new menu bar - where the button is located at the left.
Remember, for iPadOS 26, you have a configuration choice to use Full Screen Apps only, use Windowed Apps, or Stage Manager mode as you prefer:
Settings > Multitasking & Gestures - select your preferred multitasking mode

You would be well advised to review the official iPadOS User Guide and the many available online tutorials (such as YouTube) to become better acquainted with the new multitasking interface and associated gestures and controls.
Work with multiple windows at once on iPad - Apple Support
OS - iPadOS 26 - Apple
While the previous Slide Over view has been retired, Split View remains - although it is accessed differently. You can have two or three App windows side-by-side - or a spit the screen into four quadrants.
To access, assuming that the current App window is not maximised; touch-and-hold the traffic-light icon at top-left of the App window to expose the Fill & Arrange options:

The current App window will move aside, allowing you to open another App; the selected view will open the selected Apps in their own windows - which can be resized laterally, as before, by dragging the divider to left or right.
If the App window is already maximised, swipe downwards from the top-edge of the screen to expose the new menu bar - then use the traffic-light icon as described above.

The iPad User Guide is a rich and frequently overlooked resource that documents the many features and capabilities of iPad. The User Guide can be accessed here:
iPad User Guide - Apple Support
The Tips App, installed by default, provides a brief introduction to new features and capabilities of iPad and iPadOS 26 - and leads you to the User Guide. In case you missed it, here is a link to the Tips App:
https://apps.apple.com/gb/app/tips/id1069509450
Apple also has a huge YouTube Channel - containing videos and feature guidance:
https://www.youtube.com/user/Apple
[Edited by Moderator]