I've used all three engines, and have released a product using Unity. Each engine has it's strengths and weaknesses. The best advice I would give somebody is to download them and try them out and see which fits your needs. That is what we did. What attracted us to Leadwerks was it's price, render, and API. With the addition of the editor and lua scripting, the sauce keeps getting sweeter. I equally like Unity, but for different reasons; it's a good and very capable engine. I'm Not terribly impressed with Torque3D, however, from what I've read they have been making some pretty big changes and maybe someday I'll look at it again. There is not one ring to rule them all, at least not in this case.
Just my .02$
Blue