I assume that your plugins are filed in the correct Plugins folder in My PSP Files and linked via File>Preferences>File Locations.

It's a long time since I have used PSP9, so am not sure if that is ok on a Windows 7 machine.
X3 should be fine.
One thing you can try in X3 is to first make sure the plugins are in the right place, then save your current Workspace:
Customize>Workspaces>Save. Un-tick 'Include open images'.
The close X3.
Re-start it, holding the Shift key down until you get a message telling you the program will restart with the default settings. You can let go of the Shift key by then :-)
Click OK, then check if your plugins work.
Then re-load your saved workspace. I find this a great cure-all for when a file is corrupted, it clears the deck.

Good luck.

Grenou