Comment 54 for bug 1251281

@plotkin I do not know because I no longer have a copy of NX 3 that I can install, and NoMachine removed it from their site.

I would look for a config file under /etc/ where you can define what the GNOME session command is. If that doesn't work, look for an NX 3 script under /usr/ (I am not sure where they store their files) that defines what command to call, and edit it. (Assuming their license permits you to edit said scripts.)