If you did not make a backup of the old version of the app before you deleted it, there's no way you can get it back now. The current version should still work on your iPhone 3GS, though. You just need iOS 4.3 or later, so if your iPhone still has an older version, update to the latest iOS.
If, however, you really have an iPhone 3G, not a 3GS, then unless again you make a backup copy of the old version of the app, there's nothing you can do until you're ready to get a newer iPhone since iOS 4.2.1 is the final version your iPhone 3G will run.
This question, by the way, would have better been posted in the Using iPhone forum rather than here in the QuickTime forum.