Merge two APFS containers

No - there is no merging. The point of APFS is to put all your volumes in one container and not make multiple containers, but you're where you are today.

Looking at the image you should have more than enough space to move what you can off of the middle container (into the first ideally or into the last if needed. Then you can remove the middle empty container, grow the first container, then complete cleanup

Of course, just copying the data from the second two containers to another drive and then removing both, growing the first is the simplest way to return to one container with all your space.

David's comment to show the diskutil list will unlock someone giving you the exact commands needed to accomplish the rough sketch idea I've put in words here.

I would make sure that I have a current and complete backup (and, knowing me, two separate backups) of all the data from the partitions.

Then I would erase / remove all 3 partitions and start fresh from scratch.

I have had issues combining partitions in the past and clearly, based on posts here, so have others.

Starting fresh from scratch gives me the least worry solution.