undoing a partitioning

"When you delete a volume, all the data on the volume is permanently erased, and the volume is removed from the container."

Ref: Add, delete, or erase APFS volumes in Disk Utility on Mac - Apple Support.

I think there is no undo feature for Action/Commands performed in Disk Utility.

If you are currently booted to the internal drive, then it should not be possible to delete the Container holding your macOS boot volume (including the "Data" volume containing your personal files). Since only one Container (#2) has any volumes I'm guessing Container #1 and #3 are safe to delete. It all depends on what you've already done to the drive. If you made any modifications from a macOS installer, then all bets are off.

Before making any changes to a drive you should always make sure to have a good backup in case something goes wrong.

You should always have frequent and regular backups.