Levelling - How Many Tires?

I think you are asking if it is ok to level using only 1 or 2 wheels, leaving the other(s) hanging. I wouldn't feel good having a wheel/tire rated at 2000 pounds supporting 6000 pounds of RV. Raise all 3.

OTOH, by raising only 1 wheel you will raise the trailer 1/3 the thickness of the board. This is useful for fine tuning the lift. For example one 2x6 under each wheel is too little and two 2x6s is to much. One 2x6 under 1 wheel and 2 under the others may be just right.

Obvious answer is however many it takes to get level. It MAY be 3 but could also be one.

IMO, 1, 2 or 3 as long as you don't leave any tires hanging freely in the air. Make sure they're all supporting some weight.

I doubt it matters, but you may find blocking just the center one to be harder than the outside. Obviously all 3 should be on a block or the ground.

You're going to be in many situations where blocking just one or two is fine. Think of all the differences in stress and load when you're going down the road!