First of all ...read this http://www.3cx.com/blog/docs/uninstall-check/
If you are still stuck and determined to remove 3CX, and the "automated" methods won't work for you.
Try stopping the services, depending on your operating system it's usually in Start/ Control panel/Administrative Tools/services.
You should be able to pick out the 3CX services (they'll be the ones with the word 3CX in them) and Stop (don't disable) each one.
Then open Windows Explorer and find the 3Cx directories. I removed a version manually once and then re-installed. I seem to recall it was in Program Files directory, where you'd expect to find it. A lot of "stuff' is going to be left behind in the registry but should be overwritten when you go to install the next version (did you read the Blog above). It's a bit messy and obviously not the preferred method of removal.