In Windows 10 build 15019, the classic Paint got a strange update. The application now has a special button to open the Paint 3D app from classic Paint. Just a single button seems to be added.
The new button is now located at the end of the classic Paint command set on the Home tab of the Ribbon. See the following screenshot.
The integration between the two apps is not smooth. If you have some drawing opened in classic Paint, it won't be opened in Paint 3D. The button just opens the app, without opening the file you are currently editing.
It looks like the main purpose of the button is only to inform users that Paint 3D is available and can be used instead of classic Paint.
Microsoft will probably replace classic Paint with Paint 3D at some point. The company already redirected classic Paint to Paint 3D in previous builds, but later the classic Paint application was restored. See the following articles for reference: