2 Answers

Contact Apple directly:

http://www.apple.com/support/

1) Plug your iPhone 3GS into iTunes. You need to have at least iTunes 10.2.
2) Backup your device first (if you are like me, most of your purchases are direct on the iPhone.)
3) When prompted, accept the download for iOS 4.3. If you are not prompted, click on your device's name and on the Summary Tab, click Check for Updates.
Note: there are two options here, "Download and Update" or just "Download Only". The choice is up to you. I prefer to do mine in two steps.
4) The software will download and be applied to your iPhone. When completed, slide to unlock.
But make sure whether your Iphone3GS get unlocked using permanent unlocking solution. Because if you unlocked your Iphone using temporary unlocking solution it will get locked to its network when you update or upgrade it. If not unlock your Iphone using remote unlocking service from the site Theunlockarena.com ,which is a permanent unlocking solution. Then you can upgrade IOS4.3 on your Iphone3GS it won't get locked again.