For years I flew with thumbs on top of the sticks. A friend noticed and suggest that I grip the sticks between the thumb and first two fingers. I wondered what difference it would make. After flying that way for a few weeks I found that I had more precise control. This method corrected any over-control tendencies. A neck strap or TX holder is a lot of help too. It may take a bit of "getting used to" but will be better the long run. I don't get the part about the third finger behind the stick but if that's what the instructor wants do it his way. After you solo and are on your own you can do as you please.