I added a third drive earlier this year, too. FSX was already on my old D: drive, and I wanted to put it on a new larger drive.
Since I had dedicated the old D: drive mainly to FSX, to aid performance, there wasn't much else on it that would require registry changes, so I decided to do the following:
I installed the new drive as E: and then used a backup program to copy all of FSX to it.
Then, I used Control Panel->Administrative Tools->Computer Management->Disk Management to reassign the drive letters. First I changed the old D: drive to G:, then the new E: to D:, then the old G: drive to E.
After testing FSX out to see if all was well, I deleted FSX off the old drive (which was now E).
This worked perfectly. I didn't have to reinstall FSX, and had no problems with addons, since as far as XP was concerned, FSX was still on the D: drive.
Just something to consider, and of course, your mileage may vary.
Carey