I've got the longbow itch lately and was wondering what those of you that own or have shot the 3 piece thunderhorn thought of it as far as stability, quietness and speed, etc. Thanks.

Very stable, quiet and quick.  One of my brothers has 2 and his wife has 2.  I have a 2 pc. heartstopper.  Love it.  
Duane builds a great bow.

I prefer the one piece because I am a longbow guy. The three piece has a little more of a recurve feel. That being said. I have owned over 25 custom bows and Duane ranks right at the top for performance and craftsmanship.   BILL

I live about 15 miles from Duane's shop and have owned several of his Heartstoppers. All 1 piece or 2-piece though. I have shot the 3-piece on a few occasions when I've stopped in. I have never owned or seen a Thunderhorn that wasn't a work of art. Duane has an incredible eye for detail and all of his bows shoot very well. He had his own 3-piece for a long time testing and tweeking it before putting them on the market for the rest of us.

If I was in the market for a quality, well-built, easy to shoot 3-piece longbow, I would not hesitate. I think you would love it.
You don't see very many for sale used, so those that buy them must hang on to them.

P.S. I am in no way paid or endorsed by Duane, just like his bows  (though I don't own one at this time).