We can start 3 Remote Desktop sessions on Window Server 2003 without installing TS Licenses. (Two Remote Session (with mstsc) & third with mstsc /console or mstsc /admin i.e console session)
This no longer works in Window Server 2008.
It allows only to take two Remote Desktop sessions. When I go to start the third session with mstsc /admin, it ask me to disconnect one session.
How can I start a third remote session without installing TS Licenses?
You can't in 2008 and later. If you want three concurrent users, you'll have to install the Remote Desktop Services role and have the appropriate licenses.
Using that trick in 2003 was actually a violation of the license. Without buying the appropriate CALs, you were limited to two concurrent connections for server management only both then and now.