Now for some reason you need to save everything and reboot. In each instance, you choose File -> Workspace -> Save As, and save the particular workspace to a file.
When you launch TextPad, the last workspace you saved is opened.
However, on the list of recent workspaces on the File -> Workspace menu, only the last workspace you saved of the three you just created is listed.
Agreed. This is very irritating. I work on many projects simultaneously, and it's not unusual for me to have five or more TextPad sessions open at once. It's then, of course, that the mills of Windows begin to grind exceeding slow or it demands to be restarted so that it can install a patch for another bug.
The only solution I know of (other than by editing the values of the keys in HKCU\Software\Helios\TextPad 4\Recent File List in the registry) is to open each workspace in turn in one TextPad session, and make sure that that's the last one you close.