Are there any common vulnerabilities when creating a DHCP,DNS, and Active Directory server that should be addressed immediately?
The operating System is Windows Server 2012 R2 64-bit.
Well of course first and foremost is patching, obviously. Make sure that you're up to date on all security patches, and this is a critical server to keep patched.
Second, perhaps also obviously, is to limit the server to only the services and open ports required for its specific duties. Nothing extraneous should be running on this box.
Third, and perhaps most useful, is to review and apply the Microsoft Security Baselines for Windows Server 2012 R2 for the domain controller configuration. This is going to help ensure that you're configuring the server in a reasonably optimally secure manner.