Remote Access to Windows server 2012R2

You could try using the 2 admin RDP connections and set up something like Teamviever or something like it to the console session.  As long as the 2 RDP clients, aren't using the console connection, you should have 3 connections available.

Everyone wants cost effective... but if you want USERS to access the server via Remote Desktop, you cannot use the admin only connections.  Those are for ADMINISTRATION only.

You would need to enable Remote Desktop for users to access, purchase RDS Client Access Licenses (odds are, USER based licenses would be the most economical; though it depends on where the remote terminals are and who is accessing it - you didn't specify).  

Note: certain servers SHOULD NOT have RDS enabled!  You haven't provided details on what the server does for us to know if it's appropriate or not to upgrade the server or install a separate RDS system.

Typical windows server using for App development and it is a stand alone server. need to access only this server simultaneously for development purposes