Once you figure out if it's right or left hand threads, use a good impact wrench.
I went through the same thing on a Landpride mower. I tried everything I had. I borrowed an impact wrench from a friend but they wouldn't budge. So I got to thinking (right or left hand threads?). I called Landpride and found out they were right hand threads, just as I had thought. To make a long story short, I borrowed a better impact wrench. It took me maybe 5 minutes to change all 3 blades.
Needless to say, I bought a decent impact wrench /forums/images/graemlins/grin.gif