Ran that and result was exactly as you show with David shown as administrator
And did it show three separate listings for "Administrator" as you suggest in your OP.
Did it list All users as well.
I'm not sure what your actual end goal might be, but no you cannot "merge" accounts.
I posed three questions and you answered none.
I don't know or understand where you are seeing that you have three Administrator users.
IF you are simply talking about the title of some folders in C:\Users then those are profile folders, but even those cannot be named identically.
So at this point, knowing practically nothing, I would rather give you no information than bad information. Messing with actual user accounts, especially admin user accounts can be extremely detrimental to your ability to use the computer.