Login failed for user 'domain\Administrator'. (Microsoft SQL Server, Error: 18456)
I reinstalled the sql server three times.


During the installation you are asked what users to be added as instance administrators. You should add yourself. You probably added the built-in Administrators group and now you're running a LUA process and had lost the admin token and cannot access the instance. If yes, then is simply a mater of running the client process elevated ('Run as administrator').

If you have omitted to add the built-in Administrators group as SQL instance admins then you must follow the steps described at Connect to SQL Server When System Administrators Are Locked Out.