License keys for different systems: We have three servers in total (ECP, ECQ and ECT). (ECP is main server; ECQ and ECT are for testing purposes). Is it possible to generate a licence key that will work at the same time on all these servers? So that if user logs in to server ECP then GuiXT will be working. And if that user then logs in for example at server ECQ (on the same computer), GuiXT will be working even on that server, without needing a change of licence key?

The license key is specific for each SAP system name, so you will need 2 additional license keys for ECQ and ECT.
But this is not a problem for the user since you can have multiple keys in GuiXT profile (each key in a separate line) and in guixt.ini.