With 2.10.38-r1, I could rectanglular select something, ctrl-c/ctrl-v, and then push the copied object into place.
Then flatten to fix. Reasonably quick.

With 3.0.0_rc3, I need to rectangular select, ctrl-c/ctrl-v, then go to move, move the object, flatten to fix,
then go back to rectangular select in order to release, then another flatten, then it releases. Takes a lot
longer.

This is on linux.

Any way to get 3x to work like 2x?

With gimp 3.0 there are now more options in the Edit -> Paste menu. I suggest you want Edit -> Paste as Floating Data. This doesn't have a keyboard shortcut, so you can assign one using Edit -> Keyboard Shortcuts. To test this I just assigned Shift+Z so that now does what you're used to (obviously choose keys to suit you). More information on keyboard shortcuts here:
https://docs.gimp.org/3.0/en/gimp-help- ... tcuts.html

Thanks! Works well.