In addition toa and a added a new element to the dock , It now displays your latest three apps.
This new area for current apps is located between the app icons on the left side and the area on the right side that displays your open apps.
If you've set up your dock like this, you might not want this new area for current apps to fill your dock.
Hide Recent Apps in Dock
The new Recent Apps panel is so smart that it only displays newer apps that are not already docked. It can be helpful to quickly return to an app that you recently closed. However, since I've loaded my dock with all the apps I use on a regular basis, it's basically not possible to display icons for Pages, Dictionary, and Preview.
I rarely open pages, just open the dictionary randomly and just open the preview by clicking on a screenshot or a PDF. As a result, these last apps that sit in my dock day after day are not useful for my Mac workflow.
Thankfully, you can disable the feature. So it goes:
Go to System Settings> Dock and uncheck the box below for Show Current Applications in Dock .
Read more: Everything you need to know about MacOS Mojave