I want to merge the empty container disk3 to existing container disk2. How can I do it?
I think disk is both partitioned and has container inside partition Container disk2, which is in use. And second partition Container disk 3, which was created probably by accident:
So press Partition button when hard disk is selected (ST1000LM..) and choose Partition. Then select unused partition and click [-] (minus) sign.
Then select the main partition and increase its size to max:
Click apply and you are done.