I looked at these, but then decided simpler was better. I made a tri-level leveler out of a single treated 8 foot 2x6. Cost way less than any other commercial leveler and lasts just about forever. If I damage it...I'll make another. I've used the same one for about 6 years now. It works, and it's cheap.
Making it was pretty simple. I think I just cut the top board at 8 inches, the middle board at 16 inches, and the bottom board at 24 inches. I then beveled the leading edge of each board to make a "ramp" the tire drives over. Putting it together was simple enough. I aligned the un-beveled edges and screwed the 16 inch board to the 24 inch board, and then screwed the 8 inch board to the other two using exterior grade screws. that way, I have three different 1 inch (or 3/4) height adjustments. If I get a site that's more than 3 inches off level, I go to another site.
Of note, I can't take credit for this idea. I acquired it from someone else on another forum.