Grammatical Number – Hundred or Hundreds for Unit-Less Numbers?

Although all of your examples illustrate exactly the same question, for clarity's sake we'll take your examples in order:

• 3 kilograms OF oranges or 3 kilograms oranges?

3 kilograms of oranges.

In natural speech, "of" would be included. You might see it omitted in a shopping list, but never in everyday speech or writing.

• 80% OF humidity or 80% humidity?

80% humidity

"Of" is omitted in common usage. The "of" is included, though, when we reverse the order of the terms. You might hear a meteorologist predict either: "Tomorrow, we will see humidity of 80%" or "Tomorrow we will see 80% humidity."

• At the spacing OF 3 meters or at the spacing 3 meters?

At the spacing of 3 meters

Here, normal usage will include "of." A list of instructions—for instance, to a carpenter—might read:

"Height 3 meters, Width 2 meters, Spacing 3 meters."

• 100 kg OF N or 100 kg N?

100 kg of N

See "oranges" above. In narrative use, we would include "of:" "Be sure to bring 100 kilos of N!" A list of constituents, though, might read:

200 kg Ammonium Perchloride
100 kg Nitrate of Gallodinium
150 kg Phosphorus

• 30 inches OF precipitation or 30 inches precipitation?

30 inches of precipitation

The "of" will almost always be included.

• 50 meters OF height or 50 meters high?

Either of these usages may be grammatically correct, depending upon context. "High" is an adjective, and "height" is a noun, though, so this example differs from the previous ones. In normal usage, though, you would almost never see "It has 50 meters of height." Instead, one would say either:

It is 50 meters high.

Or:

It has a height of 50 meters.

It's important to understand that you will frequently see technical or scientific writing, technical support documents, instructional manuals, and lists of things, which omit "of" for brevity's sake. There is no "rule" governing such usage except that "of" should only be omitted when its omission makes a statement unclear.