Here is an article (emphasis added) from the Penn State Extension soil testing lab that answers your question regarding nitrogen:

You may notice that nitrogen (N) is not measured. Although nitrogen is a macro plant essential nutrient, required in larger quantities and is often the limiting factor in plant growth, it is very difficult to measure in the field and make accurate assumptions on how much will be available for crop use(unless it is measured in season, when plants can use it more readily). Nitrogen is a very dynamic nutrient, moving very quickly in the environment from one form to another. If we measure nitrogen when you take soil tests, typically done in the fall through early spring, the value reported doesn't mean much. By the time your crops are planted and actively growing, that pool of nitrogen measured might have leached, run-off, or left the field in another form.

The University of Minnesota form accompanying the soil test sample states something similar:

Because nitrogen is extremely mobile in soils, nitrogen recommendations are based on plant requirements and soil organic matter levels as determined by the laboratory.

The soil test facility requests that you provide two to three cups of soil for analysis; perhaps you can scrape up enough soil from your third garden to obtain close to the two cup minimum.

We hope this information is helpful to you.