How to merge "Container disk1" back into my MacintoshHD?
You did not include an image for the container holding the Macintosh HD volume. If you select Show All Devices in the View pulldown, then this container will appear. If the Physical stores for this container is disk0s2, then you can apply my answer. 

Enter the following commands.

diskutil apfs deleteContainer disk0s3
diskutil eraseVolume free none disk0s3
diskutil apfs resizeContainer disk0s2 0