Levelling - How Many Tires?

I also have a 3 axle unit and I raise all three. I do have a pack of plastic blocks to use but they don't really help enough. So far every trip we have fire wood in the back of the truck and it is split wood. I pull out three pieces that meet the height I need and use them. It keeps me from hauling more stuff and so far works perfect. I have tire jacks between the tires to keep it from rolling off. So far this method has worked perfect. I am sure there is some reason I am not supposed to use firewood but it has worked for me. I also don't get my unit perfectly level. As long as the water goes down the drain it is fine for my needs. Just a thought if you have fire wood.