Welcome to the forums -- and no need to apologise. In fact, if you take a look at other threads, your query is quite short!
Your problem does, however, sound a little strange. The SD9 camcorder uses a codec which was developed by Panasonic and which somehow or other varies slightly from the codec used by VS X2 and VS11.5+/11+. (Part of the difficulty is that the AVCHD camcorder manufacturers each seems to use their own proprietary AVCHD codec which varies slightly from the international AVCHD standard.) A lot of users with SD9 cameras had great difficulty getting their video to play back at all, let alone smoothly, in X2/11.5+, whether in Clip or Project mode. I personally could play it back smoothly in Clip mode but almost not at all in Project mode -- you can see my system specs under my Sytem button below, but essentially I have a Quad 6600.
But you say you could play your SD9 video back smoothly in Project mode in X2, but not in X3. That too is curious since others (including myself) have found the opposite i.e. the X3 seems to have corrected the X2 'bug'. And coincidentally, just yesterday, I spent several hours on projects using SD9 video.
X3 also seems to have correct another bug associated with AVCHD more generally and there is a sticky thread up towards the top of this forum on that relating to a transition blip. A fix for those continuing to have problems with that bug has been found by one of our users, which others have used quite successfully. And I am wondering if it might also work for your own problem, given that you previously had the trial version of the now old VS X2 before you installed X3.
NOTE: if you try this, make sure you keep back-up copies of the files in question so that you can restore them if the overall procedure does not work.
1) In Program Files\Common Files\Ulead Systems\MPEG took a copy of H264VDec.dll as a backup, this is the older version of the library (version 2.0.1.4)
2) Take a copy of the newer file from Program Files\Common Files\Ulead Systems\Filters\H264VDec.dll (this is version 2.0.1.7) and copy it into the folder in 1) replacing the older version there.
I might just add here that in my own case, X3 was installed on a 'virgin' computer I had just totally reformatted from the ground up with Windows 7, so it was not 'tainted' by the older installation of X2. That is perhaps why I have not had any problems with AVCHD either generally, or from the SD9 in particular when using X3.
In the meantime, all I can suggest you copy the latest version you can find and insert it in the place of any older versions you can find elsewhere, if there are any. (Again, take care to make back-ups of everything first.)