To start with, you must have your hands ahead of the ball when hitting
irons. Before I got to your second paragraph my advice was going o be to make sure your hands are ahead of the ball. Although having your hands in line with your left knee does sound a bit too far, but it is all relative to where you have your ball in your stance. Ideally, your ball should move back in your stance as your
clubs get shorter. Your 3 iron should be about where you decribed where your 6 iron is playing from. By the time you get down to your SW/LW, the ball should be somewhere in the back half of your stance. Try what is comfortable for you, but make sure your hands are ahead of the ball when hitting your
irons.
Keep us updated on your success!