The instructions assume that (in your case) you have three separate NP pages. The value gets in the TV when you put it there manually.
You use a different NP tag for each of your NPform# pages because you want them to be different. If you didn't, you'd only need one NP page. You might, for example, want to use a different &show property for each NPform.
Say you have two separate pages page1 and page2). You want page1 to be edited with NPform1 and page2 with NPform2.
The page with the NPForm1 version of the NP tag has an ID of 12.
The page with the NPForm version of the NP tag has an ID of 13.
You create the NpId Template Variable and attach it to the appropriate templates.
Then, you edit page1 and set the NpId TV to 12.
Next, you edit page2 and set the NpId TV to 13.
You add this tag to the template or the page content of page1 and page2:
[[!NpEditThisButton? &np_id=`[[*NpId]]`]]
When you click on the edit button on page1, it will be edited in the NPform1 version.
When you click on the edit button on page, it will be edited in the NPform2 version.